I have to agree with you here. Christianity is founded in the idea of original sin and the need for redemption and salvation by a good god only possible through faith that Jesus died to make that possible, without which man deserves perdition. These require the existence of an afterlife, a tri-omni deity that judges one's fitness for paradise according to conformity to specified rules of behavior described in a reliable revelation, free will, and original sin. Remove any of these and what remains isn't your version of Christianity.
Others who also consider themselves Christian find some of that to be problematic given what science has revealed. They understand that the evidence supports the scientific narrative regarding the origin and history of humanity, for example, but unlike the person able to remain a literalist in the face of that evidence, who modifies the science to conform with a literal interpretation of scripture, he modifies the meaning of scripture to comport with the science, and doesn't worry about what that implies the doctrine described above. He calls the myths allegories and doesn't worry about it the implications of that. That's how faith works.
But the problem for both groups is that they have to depart from reason somewhere. You do so by rejecting the science and accepting the myths as historical fact. Others are content to believe in some version of God and Jesus and sin and salvation without a literal Adam and Eve or original sin, which belief doesn't need to be reasonable or evidenced. In the end, one can always say that that is the mystery of creation and that God is inscrutable and beyond human understanding - that somehow it all makes sense at a higher plane and therefore doesn't need to make sense to the believer. There would be little or no cognitive dissonance there - no more than you feel. One just decides on a set of beliefs that he or she can live with and stops scrutinizing them for consistency just like the literalist does.
As for the atheistic humanist, none of this is a problem. There is no need to accept any insufficiently evidenced idea. Remove the religious dicta and the worldview that remains requires no faith to accept. It's fine with him that man evolved from prehuman forms, that consciousness might end with death, and that free will might be an illusion.