The following is an excerpt from The MacArthur New Testament Commentary on John 15.

If I had not come and spoken to them, they would not have sin, but now they have no excuse for their sin. He who hates Me hates My Father also. If I had not done among them the works which no one else did, they would not have sin; but now they have both seen and hated Me and My Father as well. (John 15:22–24)

All people are responsible for their sin, “because that which is known about God is evident within them; for God made it evident to them. For since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made, so that they are without excuse” (vv. 19–20; cf. John 1:9).

But those who heard Jesus bore an even greater responsibility for rejecting the truth. If I had not come and spoken to them, Jesus said, they would not have sin, but now they have no excuse for their sin.… If I had not done among them the works which no one else did, they would not have sin. The Lord was not speaking here of sin in general, but rather of the specific sin of willfully rejecting Him in the face of full revelation. That is the most serious sin of all, because it is the only one that is not forgivable. Having witnessed firsthand Jesus’ miracles and heard His teaching—both of which testified unmistakably to His deity (cf. Matt. 7:28–29; John 7:46; 10:25, 37–38; 14:10–11)—the Pharisees’ conclusion was, “This man casts out demons only by Beelzebul the ruler of the demons” (Matt. 12:24). Because they attributed His miraculous works to Satan instead of the Holy Spirit, Jesus pronounced their sin to be unforgivable:

Therefore I say to you, any sin and blasphemy shall be forgiven people, but blasphemy against the Spirit shall not be forgiven. Whoever speaks a word against the Son of Man, it shall be forgiven him; but whoever speaks against the Holy Spirit, it shall not be forgiven him, either in this age or in the age to come. (Matt. 12:31–32)

While that specific sin can no longer be committed, since Jesus is not physically present on earth, the principle remains the same. Total rejection in the face of total revelation is unforgivable, since there is nothing left for God to show such people. In the sobering words of the writer of Hebrews,

For in the case of those who have once been enlightened and have tasted of the heavenly gift and have been made partakers of the Holy Spirit, and have tasted the good word of God and the powers of the age to come, and then have fallen away, it is impossible to renew them again to repentance, since they again crucify to themselves the Son of God and put Him to open shame. (Heb. 6:4–6)

Despite their outward zeal (cf. Rom. 10:2), Jesus’ Jewish opponents had both seen and hated Him and the Father as well. The truth is that those who reject Christ do not know God. That applies to the one who is outwardly religious no less than it does to the hardened atheist. All false religions are of demonic origin. Paul wrote to the Corinthians, “The things which the Gentiles sacrifice, they sacrifice to demons and not to God” (1 Cor. 10:20; cf. Lev. 17:7; Deut. 32:17; Ps. 106:37). Jesus said to the most zealous religious people of His day, “You are of your father the devil, and you want to do the desires of your father” (John 8:44).