from Oscar Wilde's play
A Woman of No Importance ... Of course, this saying is coming from Illingworth, the villain of the play.
This quote is all about the good and bad of humanity, and how, as humans, we can never totally escape either. The past of the saint in the quote is implied to be a bad past, a past marked with sin. Likewise, the future of the sinner is implied to be a good future, filled with redemption.
One person cannot entirely be a saint or a sinner, at least not over the course of an entire lifetime. So maybe it's just some wishful thinking as his sinful past definitely isn't going to get any better.
In any case, you're likely to hear this one from the hard partier who wants to justify his ways. Or perhaps the questionable decision-maker who believes that
the end justifies the means.