Its just dialectics. Christ in his greatest glory assumes his greatest torment, the same way that in the apocraphyl text "book of nichademus" christ first goes to hell to then be reborn, same as in dante's inferno.
The idea of atheist christianity is the idea of christ's renunciation of gpdhood in his moment of "atheism" where matter/leviathan ultimately prevails over god - the atheism is seen by zizek in the book of job too to explain god's lack of control over nature's self-determination.
This is all very satanic obviously - but this is the symbol of the crucifix, which is capricorn, the horned god - the same emblem of catholicism.
I like the movie itself and its catholic theology, where after christ is tempted by satan he returns to his place on the cross, displaying the universality of death - or the lacking nature of god's personality.
In catholicism the idea is that the crucifix is literally the first and final event in reality, which reverberates out into time and its prophecy. This is why lacan says that the crucifix represents how God was always dead, but he just didnt know it - again, here matter takes agency over the spirit.
This notion of the dead god is also lacan's notion of the subject, as an undead no-thing, which like God, eventually comes into the reality of its absence. Or like zizek says, God must become a man for him to realise that he never existed.
This mirrors lacan's notion of the master signifier, which is also god or the absolute (given as "the name of the father", which relates not just to christianity but pagan myth), wherein the "thing" we ultimately refer back to is an absence. It is the death of god which rings out to a false memory, even to heaven; the crucifix at the heart of reality - our empty soul which fills the whole world, like the expansion of the atom.