They had not had this particular sin, of not knowing him that sent me; or they had not had such degrees of sin as they now have; or they had had more to say in excuse, or for a cover for their sin. Ignorance of the will of God will not excuse sinners wholly, but it will excuse them in part. And this last seemeth to be the sense of the words by the latter part of the verse, because it is opposed to a having no cloak nor excuse for their sin. If Christ had not come in his incarnation, in his preaching the gospel, &c, they could not have been guilty of that hatred and malice which they showed against him, which was their greatest guilt; and they would have had this to say, Lord, we knew not what Christ was, as Matthew 25:44: but now, saith our Saviour, they have no cloak, no colour, no pretence; I am come, I have revealed my Father's mind and will to them, yet they will not receive me; no, though I have done those works before them which no man ever did, nor could do but by a Divine power.
Related Commentaries of John 15:22
John 15:22
22 If I had not come and spoken unto them, they had not had sin: but now they have no clokea for their sin.