First, in their insistence on the unknowability of God: at their heart is the assertion that God is not one of the gods of religion, in the sense of a force in the universe that can't be controlled directly but only by cajolery (prayer) and bribery (sacrifice). For Aquinas, e.g., "in God there is neither genus nor species, therefore God cannot be defined"; and God is "beyond the order of all beings" (i.e, God and the universe don't add up to two).
Second, in their rejection of religion: in the world of the Israelite invention (either sense) of God, religion was the guarantor of the stratified societies ruled by priest-kings. The God of the Hebrew bible
condemns that society and that religion, e.g., in the decalogue (10 commandments), the first half of which recommends atheism in regard to the religion of the gods, and the second half of which recommends anarchism in regard to the society of the priest-kings.
For all three Abrahamic faiths, the greatest sin is, oddly enough, idolatry. They see it as acquiescing in the worship of something less than God, something that guarantees oppression. "I am YHWH your liberator, who led you out of Egypt: you shall have no gods." --CGE
Carl Remick wrote:
> ... The three Abrahamic faiths merit equal treatment as being insane
> and dangerous, not to mention impious. I don't see how anyone can claim to
> have a shred of respect for an unknowable God, yet claim allegiance to any
> of these mutually hostile faiths with their idolatrous, presumptuous boasts
> of knowing God's will. These faiths, such an endless source of woe for
> humanity, should rightly be acknowledged as the Three Stooges of the
> Apocalypse.
>
> ... I am saying that anyone who really
> believes that there is Almighty God beyond human ken cannot but find
> established religion -- with its strict, precise definitions of God --
> anything but irreligious. If you believe in God, you cannot believe
> in religion.
>