Was there anyone left on earth who knew God when Abraham was called?
No. When God called Abraham, He called him out of a whole population of pagan people, and no more knowledge of God was left on the earth. It seemed everyone had gone under the pagan influence that began at that time.
Les explains that after the Ark, God dealt with mankind differently, establishing human government to restrain murder and crime and giving permission to kill and eat animals. Later He called Abraham out of that whole population of pagan people, with no more knowledge of God left on the earth. It seemed everyone had gone under the pagan influence that began at the time of Abraham. God then turned His back on all the other nations and chose to deal with the one little nation that would come from Abraham. About 500 years later, God gave that nation yet another set of circumstances: the Law, including the Ten Commandments, the ritualistic law, the priesthood, and the temple.