15:22 “If I had not come and spoken to them, they would have no sin. But now they have no excuse for their sin” (εἰ μὴ ἦλθον καὶ ἐλάλησα αὐτοῖς, ἁμαρτίαν οὐκ εἴχοσαν· νῦν δὲ πρόφασιν οὐκ ἔχουσιν περὶ τῆς ἁμαρτίας αὐτῶν). In the second section of the pericope (vv. 22–25) Jesus transitions from the source of the world’s hatred to the judgment against the world, and transitions from his disciples’ work to his own. Jesus utters two statements that need to be understood in relation to one another: (1) “If I had not come … they would have no sin”; and (2) “now they have no excuse for their sin.” The first statement does not claim that sin only became an issue with the arrival of Jesus but rather that sin became most apparent with the arrival of Jesus (see Rom 5:13). The true source and authority for the evaluation and condemnation of sin was not the law of Moses but the law of Christ.
The second statement is best taken as synonymous with the first or as an explanation of the first. The arrival of Jesus made sin known; that is, it made it official that “now they have no excuse.” Jesus had spoken this way about judgment and sin before (see comments on 9:39–41) and how the light exposes the evil of the darkness (see comments on 3:18–20). The coming of Jesus inaugurated the judgment of the world. This is what Jesus means when he says he has taken away their “excuse” (πρόφασιν) or their “defense of an action.” As the Gospel has been making clear, Jesus is both the prosecuting attorney (Judge) and the defense attorney (sacrifice) for the world (cf. 3:16–21). One either receives the judgment they deserve from Jesus (justice and wrath) or allows Jesus to receive undeservedly the judgment on their behalf (mercy and grace).
Klink, E. W., III. (2016). John. (C. E. Arnold, Ed.) (p. 666). Grand Rapids, MI: Zondervan.