I like the explanation from Gnostic Christianity the best (though gnosticism is considered heretical by the vast majority of Christians). It seems to fill in a lot of plot holes, but I guess people that actually believe the stories as true don’t like to think about that.

The gist of it is that the God of the Old Testament is not the same a the God of the New Testament. The NT God is the true creator of the universe, and when He created the universe, He created lesser emanations of Himself. Each emanation had a divine spark within them that tied them back to God. One of these emanations, Sophia, tried her hand at creation by creating the OT God. However, this creation was a corrupt being as she was unable to instill a divine spark within it. So she hid him away from the rest of creation.

That God found Himself alone and created the world in His image and declared Himself as the one and only God. However, since he was a corrupted creation, the world He created was corrupted as well.

Sophia came clean about her mistake to the true God, so he sent her counterpart, Christ, to the Garden to try and spread the knowledge of the true reality to the humans. He created the Tree of Knowledge and took the form of a serpent to convince Eve to eat from that tree, which would give her knowledge of the corruption in the world. However, the OT God was jealous of the true God, so He cast them out and made them forget what they learned.

Later on Christ returned to Earth and sacrificed himself so that his divine spark could be set free into the world and fix the corruption that was inherent in its creation. His disciples were given the mission of spreading the knowledge of this to all of humanity.