The Essence of Christianity is a philosophical book written by Ludwig Feuerbach, containing a comprehensive critique of religion (Christianity in particular.) This book covers Feuerbach's religious arguments extensively, from the psychological origins of religion to what the idea of God represents.
Feuerbach notes some problematic ideas about the existence of God, specifically stating that the Christian conception of God cannot exist and that organized religion impacts society negatively.
He then explains what the idea of God might represent from a psychological perspective, suggesting he is a symbol of protection, safety, love, and moral guidance. This is something humans need in order to psychologically cope with the difficulties of the world, and Feuerbach argues that God is a psychological projection of these needs. However, as society has progressed and people are free to find fulfillment for these needs elsewhere, the need for God has diminished.
By projecting ideal qualities such as love and kindness onto the divine, Feuerbach argues we are not acknowledging that these are human qualities. Essentially, through religion, positive human traits are being displaced, so that humans become alienated from their own achievements. Here, we can see why Feuerbach's arguments are regarded as being humanist.
In addition to these key ideas, Feuerbach also analyses key aspects of religious thinking, such as symbolism, the Trinity, qualities commonly associated with the Christian God, and biblical stories. He analyses these key ideas under the lens of humanism, showing how human virtues are often attributed to the divine.
