Genesis 7:2

You shall take with you seven each of every clean animal, a male and his female; two each of animals that are unclean, a male and his female; (Genesis 7:2 NKJV)

Noah had to take with him on the ark every clean animal. That means, that there were clean and unclean animals before Abraham. It's possible, that God had a whole set of rules for Noah and his generation that we do not have recorded in Scripture. And when God said they were guilty, they were guilty of breaking the law. We have a different law. We live under the law of Grace. Grace is getting what you do not deserve. Which means to receive grace, we have to realize that we don't deserve it. What do you have in your life that you don't deserve?