I was saved when I was sixteen, and a few months later was watching Billy Graham having "Youth Night" on TV. Graham Said "If you lie, you can go back and tell the truth. If you steal, you can go back and repay. But if you commit immorality, you can never undo it!" He stated repeatedly that he had talked to many young people who told him that they wished they could undo their immorality but couldn't.
That teaching, that certain sins can never be undone, made a valuable impression on me that never wore off.