Albert Barnes Commentary John 16:9

"of sin, because they believe not on me;" — John 16:9 (ASV)

Of sin. The first thing specified that the world would be convinced of is sin. Sin, in general, is any violation of a law of God, but the particular sin about which people are said here to be convinced is that of rejecting the Lord Jesus. This is placed first and is considered the sin of greatest magnitude, as it is the principal one of which people are guilty.

This was particularly true of the Jews who had rejected him and crucified him. It was the great crime which, when brought home to their consciences by the preaching of the apostles, overwhelmed them with confusion and filled their hearts with remorse. It was their rejection of the Son of God that was made the great truth instrumental in their conversion (Acts 2:22, 23, 37; Acts 3:13–15; Acts 4:10, 26-28).

It is also true of other sinners. Sinners, when awakened, often feel that it has been the great crowning sin of their lives that they have rejected the tender mercy of God and trampled on the blood of his Son; and that they have for months and years refused to submit to him, saying that they would not have him reign over them.

Thus, what was spoken by Zechariah is fulfilled (Zechariah 3:10): And they shall look upon me whom they have pierced, and mourn. Throughout the New Testament, this is regarded as the sin that is preeminently offensive to God and which, if unrepented of, will certainly lead to perdition (Mark 16:16; John 3:36).

Therefore, it is placed first among those sins about which the Spirit will convince people; and therefore, if we have not yet been brought to see our guilt in rejecting God’s tender mercy through his Son, we are still in the gall of bitterness and under the bond of iniquity.