I don't sing this (I hold no religious beliefs) but I have always assumed its singers were expressing wonder and joy at their personal salvation, rather than just echoing that of an earlier reformed sinner. As such it seems to me that the exact nature of the sin would be a secondary matter. Have I simply had the wrong end of the stick?

Either way I recognise a fine song, as do many other non-believers I am sure. That I cannot subscribe to its sentiment is my loss.