Read this short document: The Devil, by Cardinal Cottier. It explains original sin well.

Some people will say, “I didn’t know it was a sin!” Saying this is no excuse since we have been created with the capacity to look for the truth and find it, as we can read in Romans 1: 18-23

For what can be known about God is evident to them, because God made it evident to them.

Ever since the creation of the world, his invisible attributes of eternal power and divinity have been able to be understood and perceived in what he has made. As a result, they have no excuse; for although they knew God they did not accord him glory as God or give him thanks. Instead, they became vain in their reasoning, and their senseless minds were darkened. While claiming to be wise, they became fools and exchanged the glory of the immortal God for the likeness of an image of mortal man or of birds or of four-legged animals or of snakes. Romans 1:18-23 (emphasis added)

“Code of Canon Law” says, “Lay persons are bound by an obligation and possess the right to acquire knowledge of Christian doctrine adapted to their capacity and condition so that they can live in accord with that doctrine.”(Canon 229, #1)

The fall by Michelangelo: Sistine Chapel