God is Not a Liar!
Think back to the scene in the Garden when God told the serpent that Eve's descendent would crush the head of the serpent's descendent. That was the first promise of Christ's victory on our behalf on the cross. Flash forward to Genesis 6 and God is so angry at the rampant sin on and by His wonderful creation that He considers wiping the slate clean. But He chose not to do that. He could have, but He didn't. Why? Because God always keeps His promises.