The most important person you've never heard of. A 19th-century German philosopher, his most important work is Die Wesen des Christentums, (The Essence of Christianity), written in 1841. In this book he lays out the basic psychological argument of atheism since that time--that God is a projection of humanity's sense of the infinite. Feuerbach argues that humans are unable to accept their own potential and so they limit themselves by projecting their ultimate potential onto an Other such as God. Instead, Feuerbach says that humanity should come to terms with its own capacity and use it, rather than wasting time worshipping a figment of its own collective imagination.

Feuerbach was a direct influence on Karl Marx and indirectly shaped the psychoanalysis and existentialism, as well as the thinking of millions of college students and would-be iconoclasts.

One neat thing Feuerbach said:

Hegel said "Man is God self-alienated" -- Feuerbach reversed this to say that "God is Man self-alienated." This more or less means that God is all the ideals Man could not achieve projected onto something until it turned into its own thing. For Feuerbach, this is a bad thing. He thought that as long as we humans continued to alienate ourselves toward some God, we could not achieve our own being.

Parts of some of this are from 'Looking at Philosophy' by Donald Palmer

One of the things I have heard throughout my life which has shocked me with a startling clarity, articulating what I have been thinking, this whole time, is something Feuerbach has said:

"We project all our unrealized perfection onto an imaginary non-human entity, God,instead of concerning ourselves with the realizable improvements of our fellow human beings".

Ludwig Feuerbach was a 19th Century German atheist theologian and philosopher of religion. One of the "Young Hegelians."

Ludwig Andreas Feuerbach (whose name means "Firebrook") was born on July 28th, 1804, in Landshut, Bavaria, the son of a jurist (I think the same Anselm von Feuerbach who was involved in the Kasper Hauser case, though I'm not entirely sure). Though baptized a Catholic, Feuerbach was educated as a Protestant and in 1822 he entered the theology school in Heidelburg under Karl Daub, where he became extremely well-versed in the writings of Luther. In 1825 he went to Berlin to study under GWF Hegel, leaving theology behind and taking up philosophy, and was awarded the title of Privatdozent in Erlangen in 1828.

In 1830 he anonymously published Thoughts on Death and Immortality, in which he denied the immortality of the soul. This controversial book was seized by the police, and cast a pall over Feuerbach's academic career; after he was associated with the text, his applications to be awarded the title of Professur were rejected three times.

In 1836, beaten but not defeated, Feuerbach withdrew into a life of private scholarship. In 1837 he married Bertha Loew, joint owner of a porcelain factory, and in 1841, the same year he published his masterpiece The Essence of Christianity, their daughter Mathilde was born; sadly, one year after he published Principles of the Philosophy of the Future, the girl dies at the age of three.

In 1848, at the behest of students, Feuerbach began lecturing on Christianity. He profoundly influenced Karl Marx and Friedrich Engels, who, although mounting thorough criticisms of Principles of the Philosophy of the Future, nonetheless had a great deal of respect for Feuerbach's humanist and materialist commitments. In 1870, he joined Marx's fledgeling Communist Party. He died two years later, on September 13, 1872, in Rechenberg, Germany.

Feuerbach perceived that, since the time of Luther, theology was no longer concerned with what God is in and of himself, but rather what He means to individuals. This marks a shift from abstraction to anthropology. However, insofar as philosophers of religion were still discussing God at all, they were perpetuating the alienation of humankind from itself. Feuerbach believed that all religious feelings were human in origin, and that God was a personification of humankind in its full potential as a species. He was quite likely the first atheist theologian. Feuerbach rejected what he perceived to be lurking theist predispositions in Hegel's thought, and a full critique of Hegel's philosophy of the future in his own book Principles of the Philosophy of the Future, though Feuerbach's presentation of his own philosophy of the future is itself a little weak and was criticized in turn by Engels and Marx – this is why he's sometimes described as the "missing link" between Hegel and Marx.

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–Feuerbach's Works:–

Thoughts on Death and Immortality (1830)
Abälard and Heloïse: The Writer and Man (1834)
The Essence of Christianity (1841)
Principles of the Philosophy of the Future (1843)

This list is not complete. Anyone who can tell me a book and a date can /msg ctf and tell me.

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Sources:
http://www.marxists.org/reference/archive/feuerbach/
http://www.cpm.ehime-u.ac.jp/AkamacHomePage/Akamac_E-text_Links/Feuerbach.html
http://www.britannica.com/eb/article?eu=34762
http://www.geschi.de/artikel/feuerbach.shtml
http://www.abaelard.de/abaelard/750feuer.htm

The are a number of problems with Ludwig's argument for atheism. The first is one of the most common errors in this kind of argument and that is the presupposition that a god must be the classical Judeo-Christian omnipotent, omniscient, infinite god. There are an amazing variety of gods out there in all shapes, sizes, and flavors who this argument doesn't necessarily apply to. This could easily be fixed by modifying his argument to consider it just against this certain type of god; however, then it would no longer be an argument for atheism but simply against the Christian type god.

Furthermore, this argument only considers one effect of the belief, that being the resultant sense of inadequacy that comes from projecting your infinite potential onto/into something else. It ignores an enourmous amount of other effects of religion; granted some of these are good while some are worse, but they must still be considered in such an argument. It even ignores the other psychological effects such as the added confidence in the belief that god is on your side

As an aside, I myself am an agnostic who feels that the whole religion thing isn't worth it most of the time; however, you can probably get a better argument against Christianity from Bertrand Russel's Why I am not a Christian, although I can't remember if he uses this type of argument along with everything else.