John the Apostle
John 16:9ESV·traditional attribution

concerning sin, because they do not believe in me;

John Calvin Reformed @genevareformer

9. Of sin. It now remains that we see what it is to convince of sin Christ appears to make unbelief the only cause of sin, and this is tortured by commentators in various ways; but, as I have already said, I do not intend to detail the opinions which have been held and advanced.

Matthew Henry Presbyterian @wholebiblehenry

As it was usual with the Old Testament prophets to comfort the church in its calamities with the promise of the Messiah (Isa 9:6; Mic 5:6; Zac 3:8); so, the Messiah being come, the promise of the Spirit was the great cordial, and is still. Three things we have here concerning the Comforter's coming: - I. That Christ's departure was absolutely necessary to the Comforter's coming, Joh 16:7.

Commenting on John 16:7-15

John Gill Reformed Baptist @doctorgill

Of sin, because they believe not on me. The "sin" here primarily intended, is that of the Jews, in disbelieving, rejecting, and crucifying Christ; and which the Spirit of God, by Peter, charged upon them on the day of "Pentecost", and fully proved against them; gave such clear evidence, and wrought such strong convictions of in their minds and consciences, that being pricked to the...