Context

It wouldn't be Oscar Wilde without plenty of barbed conversation.

This quote arrives to us in the middle of a conversation between Lady Hunstanton, Lord Illingworth, and Mrs. Arbuthnot (the titular woman of no importance), among others. The ever classy Lady Hunstanton says that she's quite out of her depth in conversations with Lord Illingworth, except for the fact that she knows he always takes the side of the sinners, and she the side of the saints. 

This quote? Illingworth's clever reply.

Where you've heard it

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. Of course, this is coming from Illingworth, the villain of the play. 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.

Pretentious Factor

If you were to drop this quote at a dinner party, would you get an in-unison "awww" or would everyone roll their eyes and never invite you back? Here it is, on a scale of 1-10.

To be fair, the actual meaning of this quote isn't pretentious at all: it's just comparing saints and sinners, saying that, given time, the two are actually very similar.

But...we can't totally let it slide: the quote does come from a play that, despite being written by the well-known Oscar Wilde, isn't super well known.