His first book, published anonymously, Gedanken über Tod und Unsterblichkeit (1830), contains an attack on personal immortality and an advocacy of the Spinozistic immortality of reabsorption in nature. These principles, combined with his embarrassed manner of public speaking, debarred him from academic advancement. After some years of struggling, during which he published his Geschichte der neueren Philosophie (2 vols., 1833 – 1837, 2nd ed. 1844), and Abelard und Heloise (1834, 3rd ed. 1877), he married in 1837 and lived a rural existence at Bruckberg near Nuremberg, supported by his wife's share in a small porcelain factory.

In two works of this period, Pierre Bayle (1838) and Philosophie und Christentum (1839), which deal largely with theology, he held that he had proven "that Christianity has in fact long vanished not only from the reason but from the life of mankind, that it is nothing more than a fixed idea." This attack is followed up in his most important work, Das Wesen des Christentums (1841), which was translated by George Eliot into English as The Essence of Christianity. "In the consciousness of the infinite, the conscious subject has for his object the infinity of his own nature."

Feuerbach's theme was a derivation of Hegel's speculative theology in which the Creation remains a part of the Creator, while the Creator remains greater than the Creation. When the student Feuerbach presented his own theory to professor Hegel, Hegel refused to reply positively to it.

In part I of his book Feuerbach developed what he calls the "true or anthropological essence of religion." Treating of God in his various aspects "as a being of the understanding," "as a moral being or law," "as love" and so on. Feuerbach talks of how man is equally a conscious being, more so than God because man has placed upon God the ability of understanding. Man contemplates many things and in doing so he becomes acquainted with himself. Feuerbach shows that in every aspect God corresponds to some feature or need of human nature. "If man is to find contentment in God," he claims, "he must find himself in God." Thus God is nothing else than man: he is, so to speak, the outward projection of man's inward nature. This projection is dubbed as a chimaera by Feuerbach, that God and the idea of a higher being is dependent upon the aspect of benevolence. Feuerbach states that, “a God who is not benevolent, not just, not wise, is no God,” and continues to say that qualities are not suddenly denoted as divine because of their godly association. The qualities themselves are divine therefore making God divine, indicating that man is capable of understanding and applying meanings of divinity to religion and not that religion makes a man divine.

The force of this attraction to religion though, giving divinity to a figure like God, is explained by Feuerbach as God is a being that acts throughout man in all forms. God, “is the principle of [man's] salvation, of [man's] good dispositions and actions, consequently [man's] own good principle and nature.” It appeals to man to give qualities to the idol of their religion because without these qualities a figure such as God would become merely an object, its importance would become obsolete, there would no longer be a feeling of an existence for God. Therefore, Feuerbach says, when man removes all qualities from God, “God is no longer anything more to him than a negative being.” Additionally, because man is imaginative, God is given traits and there holds the appeal. God is a part of man through the invention of a God. Equally though, man is repulsed by God because, “God alone is the being who acts of himself.”

In part 2 he discusses the "false or theological essence of religion," i.e., the view which regards God as having a separate existence over against man. Hence arise various mistaken beliefs, such as the belief in revelation which he believes not only injures the moral sense, but also "poisons, nay destroys, the divinest feeling in man, the sense of truth," and the belief in sacraments such as the Lord's Supper, which is to him a piece of religious materialism of which "the necessary consequences are superstition and immorality."

Part 2 comes to a crux though by seemingly retracting previous statements. Feuerbach claims that God's only action is, “the moral and eternal salvation of man: thus man has in fact no other aim than himself,” because man's actions are placed upon God. Feuerbach also contradicts himself by claiming that man gives up his personality and places it upon God who in turn is a selfish being. This selfishness turns onto man and projects man to be wicked and corrupt, that they are, “incapable of good,” and it is only God that is good, “the Good Being.” In this way Feuerbach detracts from many of his earlier assertions while showing the alienation that takes place in man by worshipping God. Feuerbach affirms that goodness is, “personified as God,” turning God into an object because if God was anything but an object nothing would need to be personified on him. The aspect of objects having previously been discussed; in that man contemplates objects and that objects themselves give conception of what externalizes man. Therefore if God is good so then should be man because God is merely an externalization of man because God is an object. However religion would show that man is inherently corrupt. Feuerbach tries to lessen his inconsistency by asking if it were possible if, “I could perceive the beauty of a fine picture if my mind were aesthetically an absolute piece of perversion?” Through Feuerbach’s reasoning it would not be possible, but it is possible, and he later states that man is capable of finding beauty.

A caustic criticism of Feuerbach was delivered in 1844 by Max Stirner. In his book Der Einzige und sein Eigentum (The Ego and His Own) he attacked Feuerbach as inconsistent in his atheism. The pertinent portions of the books, Feuerbach's reply, and Stirner's counter reply form an instructive polemics.

During the troubles of 1848 - 1849 Feuerbach's attack upon orthodoxy made him something of a hero with the revolutionary party; but he never threw himself into the political movement, and indeed lacked the qualities of a popular leader. During the period of the Frankfurt Congress he had given public lectures on religion at Heidelberg. When the diet closed he withdrew to Bruckberg and occupied himself partly with scientific study, partly with the composition of his Theogonie (1857).

In 1860 he was compelled by the failure of the porcelain factory to leave Bruckberg, and he would have suffered the extremity of want but for the assistance of friends supplemented by a public subscription. His last book, Gottheit, Freiheit und Unsterblichkeit, appeared in 1866 (2nd ed., 1890). After a long period of decline, he died on September 13, 1872. He is buried in Johannis-Friedhof Cemetery in Nuremberg, which is also where the artist Albrecht Dürer is interred.

Essentially the thought of Feuerbach consisted in a new interpretation of religion's phenomena, giving an anthropological explanation. Following Schleiermacher’s theses, Feuerbach thought religion was principally a matter of feeling in its unrestricted subjectivity. So the feeling breaks through all the limits of understanding and manifests itself in several religious beliefs. But, beyond the feeling, is the fancy, the true maker of projections of "Gods" and of the sacred in general.

Karl Marx and Friedrich Engels were strongly influenced by Feuerbach's atheism, though they criticised him for his inconsistent espousal of materialism.