Servetus was born as Miguel Serveto Conesa (family nickname "Revés") at Villanueva de Sijena, Huesca, Aragon, Spain, in 1511, probably on 29 September, the feast day of Saint Michael, although no specific record exists. Some sources give an earlier date based on Servetus' own occasional claim of having been born in 1509. His paternal ancestors came from the hamlet of Serveto, in the Aragonese Pyrenees, which gave the family their surname. The maternal line descended from Jewish conversos of the Monzón area. His father Antonio Serveto (alias Revés, i.e. "Reverse") was a notary. Servetus had two brothers: one who later became a notary like their father, and another who was a Catholic priest.

Servetus was gifted in languages and could have studied Latin, Greek and Hebrew under the instruction of Dominican friars. There is no official document that supports the contention that Servetus learned Hebrew at the Castle of Montearagón. In fact, according to expert Hebraists from Jerusalem, Servetus' Hebrew was "a Mediterranean Hebrew", one that would have been impossible to learn only from studying the Bible. It is more likely Servetus' knowledge of Hebrew results from the influence of his Jewish converso family. At the age of fifteen, Servetus entered the service of a Franciscan friar by the name of Juan de Quintana, an Erasmian. He read the entire Bible in its original languages from the manuscripts available at that time. Servetus later attended the University of Toulouse in 1526 where he studied law. There he became suspected of participating in secret meetings and activities of Protestant students.

Servetus' wide readings informed his thinking. In 1528, Servetus traveled through Germany and Italy with Quintana, who was then Charles V's confessor in the imperial retinue. In October 1530 he visited Johannes Oecolampadius in Basel, staying there for about ten months, and probably supporting himself as a proofreader for a local printer. By this time, he was already spreading his beliefs. In May 1531 he met Martin Bucer and Wolfgang Fabricius Capito in Strasbourg.

Then two months later, in July 1531, Servetus published De trinitatis erroribus ("On the Errors of the Trinity"). The next year he published Dialogorum de Trinitate ("Dialogues on the Trinity") and De Iustitia Regni Christi ("On the Justice of Christ's Reign"). He took on the pseudonym Michel de Villeneuve (i.e., "Michael from Villanueva"), to avoid persecution by the ecclesiastical authorities in power at the time because of these religious works. He studied at the Collège de Calvi in Paris in 1533. After an interval, Servetus returned to Paris to study medicine in 1536. In Paris, his teachers included Sylvius, Fernel, and Johannes Guinter, who hailed him with Vesalius as his most able assistant in dissections.

In these books, Servetus rejected the classical conception of the Trinity, stating that it was not based on the Bible. He argued that it arose from teachings of (Greek) philosophers, and he advocated a return to the simplicity of the Gospels and the teachings of the early Church Fathers that he believed pre-dated the development of Nicene trinitarianism. Servetus hoped that the dismissal of the trinitarian dogma would make Christianity more appealing to believers in Judaism and Islam, which had preserved the unity of God in their teachings. According to Servetus, trinitarians had turned Christianity into a form of "tritheism", or belief in three gods. Servetus affirmed that the divine Logos, the manifestation of God and not a separate divine Person, was incarnated in a human being, Jesus, when God's spirit came into the womb of the Virgin Mary. Only from the moment of conception, was the Son actually generated. Therefore the Son was not eternal, but only the Logos from which He was formed. For this reason, Servetus always rejected calling Christ the "eternal Son of God" but rather called him "the Son of the eternal God." In describing Servetus' view of the Logos, Andrew Dibb explained: "In 'Genesis' God reveals himself as the creator. In 'John' he reveals that he created by means of the Word, or Logos. Finally, also in 'John', he shows that this Logos became flesh and 'dwelt among us'. Creation took place by the spoken word, for God said "Let there be ..." The spoken word of Genesis, the Logos of John, and the Christ, are all one and the same."

Unitarian scholar Earl Morse Wilbur states, "Servetus' Errors of the Trinity is hardly heretical in intent, rather is suffused with passionate earnestness, warm piety, an ardent reverence for Scripture, and a love for Christ so mystical and overpowering that [he] can hardly find words to express it... Servetus asserted that the Father, Son and Holy Spirit were dispositions of God, and not separate and distinct beings. Wilbur promotes the idea that Servetus was a modalist.

Servetus states his view clearly in the preamble to Restoration of Christianity (1553): "There is nothing greater, reader, than to recognize that God has been manifested as substance, and that His divine nature has been truly communicated. We shall clearly apprehend the manifestation of God through the Word and his communication through the Spirit, both of them substantially in Christ alone." This theology, though original in some respects, has often been compared to Adoptionism, Arianism, and Sabellianism, all of which Trinitarian scholars rejected as heresies in their attempts to defend their belief that God exists eternally in three separate persons. Nevertheless, Servetus rejected these theologies in his books: Adoptionism, because it denied Jesus's divinity; Arianism, because it multiplied the hypostases and established a rank; and Sabellianism, because, at first glance, it seemingly confused the Father with the Son.

The incomprehensible God is known through Christ, by faith, rather than by philosophical speculations. He manifests God to us, being the expression of His very being, and through him alone, God can be known. The scriptures reveal Him to those who have faith; and thus we come to know the Holy Spirit as the Divine impulse within us.

Under severe pressure from Catholics and Protestants alike, Servetus clarified this explanation in his second book, Dialogues (1532), to show the Logos coterminous with Christ. He was nevertheless accused of heresy because of his insistence on denying the dogma of the Trinity and the individuality of three divine Persons in one God.

After his studies in medicine, Servetus started a medical practice. He became personal physician to Pierre Palmier, Archbishop of Vienne, and was also physician to Guy de Maugiron, the lieutenant governor of Dauphiné. While he practiced medicine near Lyon for about fifteen years, he also published two other works dealing with Ptolemy's Geography. Servetus dedicated his first edition of Ptolemy and his edition of the Bible to his patron Hugues de la Porte. He dedicated his second edition of Ptolemy's Geography to his other patron, Archbishop Palmier. While in Lyon, Symphorien Champier, a medical humanist, had been Servetus' patron. Servetus wrote pharmacological tracts in defense of Champier against Leonhart Fuchs. Working also as a proof reader, he published several more books which dealt with medicine and pharmacology. Years earlier he had sent a copy to John Calvin, initiating a correspondence between the two. Initially in the correspondence Servetus used the pseudonym "Michel de Villeneuve".

In 1553 Servetus published yet another religious work with further anti - trinitarian views. It was entitled Christianismi Restitutio, a work that sharply rejected the idea of predestination and the idea that God condemned souls to Hell regardless of worth or merit. God, insisted Servetus, condemns no one who does not condemn himself through thought, word or deed.

To Calvin, who had written his summary of Christian doctrine Institutes of the Christian Religion, Servetus' latest book was an attack on his personally held theories regarding Christian belief, theories that he was attempting to put forth as "established Christian doctrine". Calvin sent a copy of his own book as his reply. Servetus promptly returned it, thoroughly annotated with critical observations. Calvin wrote to Servetus, "I neither hate you nor despise you; nor do I wish to persecute you; but I would be as hard as iron when I behold you insulting sound doctrine with so great audacity."

In time their correspondences grew more heated until Calvin ended it. Servetus sent Calvin several more letters, to which Calvin took offense. Thus, Calvin's antagonism against Servetus seems to have been based not simply on his views but also on Servetus's tone, which he considered inappropriate. Calvin revealed the intentions of his offended heart when he stated of Servetus, when writing to his friend William Farel on 13 February 1546:

On 16 February 1553, Servetus, while in Vienne, was denounced as a heretic by Guillaume Trie, a rich merchant who had taken refuge in Geneva — a very good friend of Calvin, — in a letter sent to a cousin, Antoine Arneys, who was living in Lyon. On behalf of the French inquisitor Matthieu Ory, Servetus as well as Arnollet, the printer of Christianismi Restitutio, were questioned, but they denied all charges and were released for lack of evidence. Arneys was asked by Ory to write back to Trie, demanding proof. On 26 March 1553, the letters sent by Servetus to Calvin and some manuscript pages of Christianismi Restitutio were forwarded to Lyon by Trie. On 4 April 1553 Servetus was arrested by Roman Catholic authorities, and imprisoned in Vienne. He escaped from prison three days later. On 17 June, he was convicted of heresy by the French Inquisition, "thanks to the 17 letters sent by Jehan Calvin, preacher in Geneva" and sentenced to be burned with his books. An effigy and his books were burned in his absence.

Meaning to flee to Italy, Servetus inexplicably stopped in at Geneva, where Calvin and his Reformers had denounced him. On 13 August, he attended a sermon by Calvin at Geneva. He was immediately recognized and arrested after the service and was again imprisoned. All his property was confiscated. French Inquisitors asked that Servetus be extradited to them for execution. Calvin wanted to show himself as firm in defense of Christian orthodoxy as his usual opponents. "He was forced to push the condemnation of Servetus with all the means at his command." Calvin's delicate health meant he did not personally appear against Servetus. Nicholas de la Fontaine played the more active role in Servetus's prosecution and the listing of points that condemned him.

At his trial, Servetus was condemned on two counts, for spreading and preaching Nontrinitarianism and anti - paedobaptism (anti - infant baptism). Of paedobaptism Servetus had said, "It is an invention of the devil, an infernal falsity for the destruction of all Christianity." In the case the procureur général (chief public prosecutor) added some curious sounding accusations in the form of inquiries — the most odd sounding perhaps being, "whether he has married, and if he answers that he has not, he shall be asked why, in consideration of his age, he could refrain so long from marriage." To this oblique imputation of unchastity (or perhaps homosexuality), Servetus replied that rupture had long since made him incapable of that particular sin. More offensive to modern ears might be the question "whether he did not know that his doctrine was pernicious, considering that he favours Jews and Turks, by making excuses for them, and if he has not studied the Koran in order to disprove and controvert the doctrine and religion that the Christian churches hold, together with other profane books, from which people ought to abstain in matters of religion, according to the doctrine of St. Paul."

Calvin believed Servetus deserving of death on account of what he termed as his "execrable blasphemies". Calvin expressed these sentiments in a letter to Farel, written about a week after Servetus’ arrest, in which he also mentioned an exchange with Servetus. Calvin wrote:

As Servetus was not a citizen of Geneva, and legally could at worst be banished, the government, in an attempt to find some plausible excuse to disregard this legal reality, had consulted with other Swiss Reformed cantons (Zürich, Bern, Basel, Schaffhausen.) They universally favored his condemnation and suppression of his doctrine, but without saying how that should be accomplished. Martin Luther had condemned his writing in strong terms. Servetus and Philip Melanchthon had strongly hostile views of each other. The party called the "Libertines", who were generally opposed to anything and everything John Calvin supported, were in this case strongly in favor of the execution of Servetus at the stake (while Calvin argued for something more humane). In fact, the council that condemned Servetus was presided over by Perrin (a Libertine) who ultimately on 24 October sentenced Servetus to death by burning for denying the Trinity and infant baptism. When Calvin requested that Servetus be executed by decapitation rather than fire, Farel, in a letter of 8 September, chided him for undue lenience. The Geneva Council refused his request. On 27 October 1553 Servetus was burned at the stake just outside Geneva with what was believed to be the last copy of his book chained to his leg. Historians record his last words as: "Jesus, Son of the Eternal God, have mercy on me."

Calvin agreed that those whom the ruling religious authorities determined to be heretics should be punished:

Sebastian Castellio and countless others denounced this execution and became harsh critics of Calvin because of the whole affair.

Some other antitrinitarian thinkers began to be more cautious in expressing their views: Martin Cellarius, Lelio Sozzini and others either ceased writing or wrote only in private. The fact that Servetus was dead meant that his writings could be distributed more liberally, though others such as Giorgio Biandrata developed them in their own names.

The writings of Servetus had some influence on the beginnings of the Unitarian movement in Poland and Transylvania. Piotr z Goniądza's advocacy of Servetus' views led to the separation of the Polish brethren from the Calvinist Reformed Church in Poland, and laid the foundations for the Socinian movement which fostered the early Unitarians in England like John Biddle.

Because of his rejection of the Trinity and eventual execution by burning for heresy, Unitarians often regard Servetus as the first (modern) Unitarian martyr - though he was not a Unitarian either in the 17th C sense nor the modern one. Other non - trinitarian groups, such as Jehovah's Witnesses, and Oneness Pentecostalism, also claim Servetus as a spiritual ancestor. Oneness Pentecostalism particularly identifies with Servetus' teaching on the divinity of Jesus Christ and his insistence on the oneness of God, rather than a Trinity of three distinct persons: "And because His Spirit was wholly God He is called God, just as from His flesh He is called man."

Swedenborg wrote a systematic theology that had many similarities to the theology of Servetus.

More recently Servetus' name has been given prominence by the originally anonymous author "Servetus the Evangelical".

In recent years Michael Servetus has also been credited with being one of the modern forerunners of freedom of religion and freedom of conscience in the Western world. A renowned Spanish scholar on Servetus' work, Ángel Alcalá, identified the radical search for truth and the right for freedom of conscience as Servetus' main legacies, rather than his theology. The Polish - American scholar, Marian Hillar, has studied the evolution of freedom of conscience, from Servetus and the Polish Socinians, to John Locke and to Thomas Jefferson and the American Declaration of Independence. According to Hillar: "Historically speaking, Servetus died so that freedom of conscience could become a civil right in modern society."

In 1903 latter - day followers of Calvin erected a plaque at the site of Servetus' death, "repudiating his [Calvin's] mistake which was the mistake of his age."

Servetus was the first European to describe the function of pulmonary circulation, although his achievement was not widely recognized at the time, for a few reasons. One was that the description appeared in a theological treatise, Christianismi Restitutio, not in a book on medicine. Most copies of the book were burned shortly after its publication in 1553 because of persecution of Servetus by religious authorities. Three copies survived, but these remained hidden for decades.

It was not until William Harvey's publication of findings from his dissections in 1616 that the function of pulmonary circulation was widely accepted by physicians. Western historians of science have since learned that a separate discovery of pulmonary circulation was made in the 13th century by Ibn al - Nafis, who was born in 1213 A.D. in Damascus. Because of limitations of culture and language, his discovery was not transmitted to Europe.

In 1984, a Zaragoza public hospital changed its name from José Antonio to Miguel Servet. It is now a university hospital. Most Spanish cities also include at least a street, square or park named after Servetus. Geneva, Switzerland, named a street after him: la rue Michel Servet.

Increasing numbers of Servet Scholars agree that previously unknown works have been found by Dr Francisco Javier González Echeverría, a Spanish pediatrician and historian of the modern age. These works include a noted "Dioscorides" found at Sesma, a village of Navarra, and several bibles.

• Austrian author Stefan Zweig features Servetus in Castellio gegen Calvin oder Ein Gewissen gegen die Gewalt.
• Canadian dramatist Robert Lalonde wrote Vesalius and Servetus, a 2008 play on Servetus.