They are wrong about sin is literally “concerning sin.” Here, as elsewhere in the Gospel of John, the cardinal sin of the world is the refusal to believe in Jesus. Although the Greek simply has “do not believe,” it is legitimate to make the idea of “refusal” specific, as New English Bible, Jerusalem Bible, and New American Bible do. This verse may, therefore, be rendered in some languages “they are wrong about what sin is, because they refuse to put their trust in me.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on the Gospel of John. (UBS Handbook Series). New York: UBS, 1980. For this and other handbooks for translators see here .