22 If I had not come and spoken to them, they would have no sin, but now they have no excuse for their sin.
The Lord now proceeded to show why people hate Him and His followers: (1) the Lord exposed the sin of these haters (Jn 15:22,24) and (2) they did not know the Father or His revelation of His Son (Jn 15:21, 23, 24b; 16:3).
22 If [assumed not true] I had not come [in His incarnation] and spoken to them,
The “if” here assumes the statement not to be true (second class condition). Jesus did in fact come and speak to Israel.
they would have no sin,
The idea is they would have no sin, but, in fact, they do. Sin would not be so stark without the revelation that Jesus brought to the world. This is a hypothetical point that our Lord made here because the Holy Spirit would come to convict the world of its sin (Jn 16:9). Jesus was not saying that they would be without sin in every sense of that idea but in the category of negative volition toward Him.
but now they have no excuse [pretext] for their sin.
In comparison to previous revelations of God’s plan, Jesus’ revelation made all previous exposures to God dim by comparison. People would now have no rationalization for their sin.
Now that Jesus has come, no one has an excuse to reject Him. The sin of rejecting God is clearly imputed to those who reject Christ, because He is the stark revelation of God. There is now no way to escape from the truth that Jesus presents.
Light brings responsibility.
Since Jesus came to reveal the light of the nature of God, people have no excuse for rejecting His plan. Jesus robbed people of any excuse for their sin, the specific sin of not knowing God or the revelation of His Logos. People will not recognize the Father’s perfect and enormous revelation in Christ.
The greatest sin a person can commit is rejection of Jesus as Savior. All other sins pale in comparison to that. Jesus is the true revelation of God. There is no justification for rejection of Him.
Had Jesus not revealed Himself, people would not be culpable before God. However, He has revealed Himself, so everyone is culpable about that knowledge. Now people are responsible for rejecting Him as their Savior. No one has an excuse for doing this. There is no justification for rejecting Jesus as the means to go to heaven.
People might plead ignorance as an excuse for their sin (Ac 17:30). The “Light” has come into the world to reveal the nature of God and the problem that causes for men’s eternal future. This means no one has an excuse to reject Jesus or His message (Jn 3:19,20).