“Then love is sin, and let me sinful be.”

1780s, Annotations to Lavater (1788)

Source: Discipleship (1937), The Disciple and Unbelievers, p. 184.

Source: The Kite Runner

Book II, ch. 3 (trans. Constance Garnett)
The Elder Zossima, speaking to a devout widow afraid of death
The Brothers Karamazov (1879–1880)

Original: ​(it) In amore, peccate con il cuore.
Source: prevale.net

The Thirteenth Revelation, Chapter 40
Context: But now if any man or woman because of all this spiritual comfort that is aforesaid, be stirred by folly to say or to think: If this be true, then were it good to sin to have the more meed, — or else to charge the less to sin, — beware of this stirring: for verily if it come it is untrue, and of the enemy of the same true love that teacheth us that we should hate sin only for love. I am sure by mine own feeling, the more that any kind soul seeth this in the courteous love of our Lord God, the lother he is to sin and the more he is ashamed. For if afore us were laid all the pains in Hell and in Purgatory and in Earth — death and other —, and sin, we should rather choose all that pain than sin. For sin is so vile and so greatly to be hated that it may be likened to no pain which is not sin. And to me was shewed no harder hell than sin. For a kind soul hath no hell but sin.

Source: Little Women

Vogue, 1990

This is variant of a traditional Christian proverb; ie: "Hate the sin, but love the sinner" in Sermons, Lectures, and Occasional Discourses (1828) Edward Irving http://books.google.co.uk/books?id=VwUeH-wTxZ8C&pg=PA132, and similar expressions date to those of Augustine of Hippo: "Love the sinner and hate the sin." Gandhi did express approval of such sentiments in his An Autobiography (1927): "Hate the sin and not the sinner" is a precept which, though easy enough to understand, is rarely practiced, and that is why the poison of hatred spreads in the world.
Misattributed