Was there any idolatry or organized religion before the Flood?
No. Before the Flood there was no instance of idolatry, no religion of sorts, no temple worship, and no organized worship. After the Flood, eight people who knew God multiplied, and within about 200 years they gathered at the Tower of Babel, where pagan idolatry began.
Les explains that before the Flood there was never an instance of idolatry, no religion of sorts, no temple worship of any kind, and no organized worship. Man became corrupt and violent because there was nothing to control his thinking. After the Flood, Noah, his wife, his three sons, and their wives—eight people with a knowledge of God—multiplied. Within about 200 years, only a generation or two, that expanding population gathered around the Tower of Babel, ready to embrace pagan worship. Nimrod, the first world totalitarian despot and hunter of men, was setting up a religion reaching to heaven, and that is where the beginning of all pagan idolatry lies.