For those who believe there was no historical Adam and Eve, how does Christianity make sense?

How can Christianity make sense without a literal Adam and Eve? I have heard many Christians (particularly Catholics) try and be conciliatory with modern science by claiming that Adam and Eve were not real people and that Genesis was something along the lines of an allegory. But how does this make sense from a theological perspective? Wasn’t the entire point of Christ’s atonement to redeem humanity from their fall from grace, which is indeed the fault of humanity precisely because of Adam and Eve? How can the theologically “progressive” Christian explain the origin of sin, humanity’s state of sin, and the purpose of Christ’s atonement without resorting to Adam and Eve?