Are you unguilty of sin if you never heard the gospel?

The sin Jesus speaks of here is the sin of rejecting Him as the Christ. Jesus is condemning the people of the world who hate Him and warning his disciples that the world will hate them also because they bear His name (John 15:18-21).

If God had not sent Him into the world, they would not have been guilty of rejecting Him. If they had not seen His miracles, they would not have been guilty of ascribing His power to Beelzebub, the prince of demons (Luke 11:14-20).

Here is an extract from a sermon delivered on 16 May, 1858 by the Rev. C. H. Spurgeon on human responsibility, based on John 15:22: “If I had not come and spoken to them, they had not had sin: but now they have no cloak for their sin.”

THE peculiar sin of the Jews, the sin which aggravated above everything their former iniquities, was their rejection of Jesus Christ as the Messiah. He had been very plainly described in the books of the prophets, and they who waited for Him, such as Simeon and Anna, no sooner beheld Him, even in His infant state, than they rejoiced to see Him, and understood that God had sent forth His salvation. But because Jesus Christ did not answer the expectation of that evil generation, because He did not come arrayed in pomp and clothed with power, because He had not the outward garnishing of a prince and the honors of a king, they shut their eyes against Him, He was “a root out of a dry ground,” He was “despised and they esteemed Him not.” https://www.spurgeongems.org/sermon/chs194.pdf

It’s a lengthy sermon (8 full pages) and in the final section Spurgeon warns his audience of the sin of rejecting Christ having heard the Gospel. But when Jesus spoke his words 2,000 years ago, he did not actually say that the Jews (especially the religious leaders) were guilty of sin because they had never heard the Gospel. The sin they were guilty of was of rejecting Jesus. The Gospel message had not yet been released to the world – that would not happen till after Jesus’ death, resurrection and the arrival of the Holy Spirit at Pentecost.

Context is everything! This belief is in accord with Reformed Protestant theology.

As a Protestant of the Reformed faith, I would point to the biblical understanding that "all have sinned and fall short of the glory of God" (Romans 3:9 & 23) whether or not they have heard the gospel message about Jesus Christ. Sin abounded in the world for thousands of years before the Son of God came into the world as the man, Jesus, who is - in his person - the good news (or gospel). "Wherefore, as by one man [Adam] sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned: (For until the law sin was in the world: but sin is not imputed when there is no law." (Romans 5:12-13)

And that mention of the law is the crux of the matter. Paul goes on to say that "I had not known sin, except the law had said, 'thou shalt not covet'... For without the law, sin was dead." (Romans 7:7-8) It is the law that condemns us all as sinners, for we are all law-breakers, even those who have never heard the good news about Jesus.

Paul further explains this in Galatians 3:17-25 where he describes the law as a "schoolmaster" to bring us to Christ. The purpose of the law is to bring us to miracle of being justified by faith, once we stop striving to keep law as a means of pleasing God, but rest entirely in the finished work of Christ on the cross to be justified - by faith - not by law-keeping or any other kind of works. Once Christians get to that transformed state, they are no longer under the schoolmaster of law. They are liberated as children of God who have both understood and believed the gospel.

Those who heard Jesus and witnessed his miracles, were under law, which law told them to listen to the prophet greater than Moses who was to come into their midst as the Messiah, but they rejected Christ as that Messiah. They preferred to stick to the legalistic course of law-keeping and that is what blinded them to Jesus being, in his person, the good news, the gospel of grace.