Dying for what one believes is proof of sincerity, but by itself it does not prove the truth of the martyr's belief. However, the death of the first martyrs for Christ is a proof of the truth of that for which they suffered and died. First, they knew whether or not what they taught was true. They knew they had been with Christ, that they had seen His miracles, that they had observed His sinless life, that they had pondered His matchless teaching, that He had died on the cross, that He had been buried, and that within a short time they had seen Him alive. They had scientific proof that they were again with Jesus. This proof came by the seeing of the eye, as they saw His familiar features, by the hearing of the ear, as they heard His beloved voice, and by the impact on them of His well known personality. They touched Him, they ate with Him, they heard Him teach them, they saw Him under a variety of circumstances, and when more than five hundred persons were present. (I Cor. 15:5-8). It was impossible for so many to have been deceived so many times. They knew that Christ had been raised from the dead and they demonstrated their sincerity by dying for this great truth. Theirs was not just a subjective experience. Once we accept the truth of the resurrection, to which they bore witness, it is easy to accept the other miracles of Jesus and to accept the truth of His teaching.
Paul, who was not with Christ in His personal ministry, knew whether he had seen the Lord, whether the Lord appeared to him at other times, whether his experience was corroborated by the Lord's instructions to Ananias, whether he had learned the gospel directly from Jesus and not from men, whether he taught the same message the other apostles taught, whether he worked a wide variety of miracles, and whether he was able to lay hands on others and confer miraculous gifts.
Second, these martyrs knew that Christ had not promised them, and events proved they would not receive, earthly power, riches, or the satisfaction of lusts as a result of the message which they preached. On the other hand, they knew that preaching Christ could result in the loss of all earthly gain and of life itself. How did these first martyrs know it could cost everything if they preached Christ? Jesus told them that persecution would come. (Matthew 5:10-12; John 15:20-21; 16:1-3). They knew Christ had been persecuted unto death. They knew that to preach the risen Christ would arouse the wrath of the ones who had taken the lead in having Jesus put to death. They would not only be telling these men that they were wrong, but they would be laying the responsibility for the blood of Jesus at their door, and be affirming that Jesus was the Messiah to the very people who thought that the rejection and death of Jesus was proof that He was not the Messiah. (Acts 5:28). Furthermore, the book of Acts shows they had not been preaching long before they were persecuted.
Third, these martyrs did not die in carnal warfare in an effort to enslave others, but died in an effort to bring the message of salvation even to those who persecuted and finally killed them.
Is it not clear that we should accept the testimony of the first martyrs of Jesus Christ?