1 The Lord had said above to His disciples, “If they have persecuted me, they will also persecute you; if they have kept my saying, they will keep yours also. But all these things will they do unto you for my name's sake, because they know not Him that sent me.” And if we inquire of whom He so spoke, we find that He was led on to these words from what He had said before, “If the world hate you, know ye that it hated me before [it hated] you;” and now in adding, “If I had not come and spoken unto them, they had not had sin,” He more expressly pointed to the Jews.
Of them, therefore, He also uttered the words that precede, for so does the context itself imply. For it is of the same parties that He said, “If I had not come and spoken unto them, they had not had sin;” of whom He also said, “If they have persecuted me, they will also persecute you; if they have kept my saying, they will keep yours also; but all these things will they do unto you for my name's sake, because they know not Him that sent me;” for it is to these words that He also subjoins the following: “If I had not come and spoken unto them, they had not had sin.”
The Jews, therefore, persecuted Christ, as the Gospel very clearly indicates, and Christ spoke to the Jews, not to other nations; and it is they, therefore, that He meant to be understood by the world, that hates Christ and His disciples; and, indeed, not those alone, but even these latter were shown by Him to belong to the same world. What, then, does He mean by the words, “If I had not come and spoken unto them, they had not had sin”? Was it that the Jews were without sin before Christ came to them in the flesh?
Who, though he were the greatest fool, would say so? But it is some great sin, and not every sin, that He would have to be understood, as it were, under the general designation. For this is the sin wherein all sins are included; and whosoever is free from it, has all his sins forgiven him: and this it is, that they believed not on Christ, who came for the very purpose of enlisting their faith. From this sin, had He not come, they would certainly have been free. His advent has become as much fraught with destruction to unbelievers, as it is with salvation to those that believe; for He, the Head and Prince of the apostles, has Himself, as it were, become what they declared of themselves, “to some, indeed, the savour of life unto life; and to some the savor of death unto death.”
Source: Tractates on the Gospel of John (New Advent)