How can a Christian reject part of the Bible and still call themselves a Christian?

[Sword of Christ]

Seeing as the only reason we're in "sin" is because Yahweh decided Adam and Eve's punishment would carry on into people who had no control over it, and some of the things he likes and dislikes are things many people would be against just through common sense, I don't really care much if he thinks I'm "sinning" or not.

The story of Genesis is just a symbolic/poetic way of describing the human condition in relationship to a perfect/flawless creator God. We sin because we have our independent freewill from God as physical beings. Because we're physical beings we're subject to suffering and death. Sin severs you from God meaning your relationship with God ceases on death, you're in the ground and that's the end of you much as the atheists believe. However what they wrote at the time they only had half of the story. In the completed edition God cleanses your sin away by grace and restores you to full life and this way you become an immortal being while retaining your individual freedom of will. Jesus provided this part of the narrative/revelation from God so completing the picture as we understand it. They call it the Good News for a reason.

It will die down quite a bit unless it goes with the flow of science and reality, though.

Science is the study of the universe and God created the universe science is then able to study so that gives us the full understanding of science and reality. The puzzle pieces fit together to form the whole picture.