I recently saw a skeptic in an online forum raise a popular objection to a Christian argument for Jesus' resurrection. He asked for early sources claiming that the apostles died for their testimony to the resurrection of Christ. Apparently, what he wanted people to conclude is that we don't have any credible accounts of any of the apostles having died as a result of their belief in the resurrection. Thus, the popular Christian appeal to the apostles' willingness to die for their testimony about the resurrection can't be verified.
There are some elements of truth to this objection. Some of the accounts of the apostles' deaths come from sources who lived long after the time of the apostles. We have no account of the death of Thomas or the death of Andrew in the New Testament or in the earliest patristic sources, for example. And the early accounts of the deaths of Paul and Peter, for example, don't refer to their dying for a belief in the resurrection.
But the elements of truth in this skeptical objection are accompanied by a lot of bad reasoning. Why limit the question to the apostles? Why not include other early Christians as well? And why only include martyrdom? Why not suffering in general, such as imprisonment? Why would the early accounts of Christian suffering have to be about individuals, such as how an individual apostle died? Why can't we include general descriptions of the sufferings of the apostles and other early Christians, without an individual apostle having to be singled out? And how realistic is it to expect some sort of report of an apostle dying for a particular doctrine, like the resurrection? The early Christians were persecuted for a variety of reasons, and there's no need for the resurrection to be singled out in order for us to reasonably conclude that the resurrection was one of the factors involved.
One of the questions we ought to ask is whether the resurrection was just a minor aspect of the early Christian faith or a major aspect of it. Is it likely that people were suffering as Christians without much concern for or confidence in the resurrection?
All four gospels present the resurrection as a vindication of Jesus, as a highly significant event that allows Jesus to complete His work on earth and thus end the gospel account. Would anybody honestly and thoughtfully reading the four gospels conclude that the resurrection is only a minor element of the Christian faith in any of the four accounts? The resurrection is foundational in Acts as well, presented as a vindication of Jesus in the same manner as the gospels (Acts 2:31-32, 13:30-37, 17:31). On one occasion, Paul refers to how he was suffering as a result of his belief in the resurrection (Acts 23:6). In 1 Corinthians 15:3-11, Paul refers to the resurrection as something "of first importance" (verse 3) that was taught by all of the apostles (verse 11). Paul goes on to refer to how Christian faith is worthless and sinful without Jesus' resurrection (1 Corinthians 15:12-19). Peter similarly refers to the resurrection as foundational (1 Peter 1:3). Etc. When the early Christians suffered, they suffered for a faith that had the resurrection as one of its most foundational elements.
While it's true that we don't have first or second generation accounts of the martyrdom of somebody like the apostle Philip or the apostle Andrew, we do have a lot of early evidence for Christian suffering in general and the martyrdom of some apostles. Paul referred to the suffering of the apostles in general (1 Corinthians 4:9). Men like Paul and John suffered imprisonment (Philippians 1:13, Revelation 1:9) and suffered for their Christian beliefs in other ways (2 Corinthians 11:23-33). The New Testament refers to the martyrdom of James (Acts 12:2) and Peter (John 21:18-19) and implies the martyrdom of Paul (2 Timothy 4:6-8). The first century historian Josephus refers to the martyrdom of Jesus' brother, James (Antiquities, 20:9). Clement of Rome, writing in the late first century, implies the martyrdom of Peter and Paul (First Clement, 5). Though Clement only names Peter and Paul, he refers to "the illustrious apostles", so he may have had more than those two apostles in mind, even though he only named those two as examples.
Apparently writing near the end of the first century, after at least most of the other apostles had died, John refers to the apostles in general as faithful witnesses to Christ (Revelation 21:14). If any of the apostles (or the brothers of Jesus, for example) had renounced the faith, neither John nor any other early source seems to be aware of it. You would think that if any of the apostles, brothers of Jesus, or other prominent early Christian leaders had gone the way of Judas, we would hear about it. We would expect anybody who went the way of Judas to have a reputation that went the way of Judas' reputation. But we don't see people like the apostles and Jesus' brothers treated as Judas is treated in the early sources. Polycarp, in the early second century, writes of the apostles in general:
"I exhort you all, therefore, to yield obedience to the word of righteousness, and to exercise all patience, such as ye have seen set before your eyes, not only in the case of the blessed Ignatius, and Zosimus, and Rufus, but also in others among yourselves, and in Paul himself, and the rest of the apostles. This do in the assurance that all these have not run in vain, but in faith and righteousness, and that they are now in their due place in the presence of the Lord, with whom also they suffered. For they loved not this present world, but Him who died for us, and for our sakes was raised again by God from the dead." (Letter to the Philippians, 9)
Similarly, other men who were contemporaries of the apostles write about the unity they had and refer to them collectively as if they had taught the same doctrines: Clement of Rome (First Clement 5, 42, 44); Ignatius (Letter to the Ephesians, 11; Letter to the Magnesians, 13; Letter to the Romans. 4); Aristides (Apology, 2); The Epistle of Barnabas (5); etc. Papias, who probably was a disciple of John (Irenaeus, Against Heresies, 5:33:4), wrote:
"If, then, any one came, who had been a follower of the elders, I questioned him in regard to the words of the elders, - what Andrew or what Peter said, or what was said by Philip, or by Thomas, or by James, or by John, or by Matthew, or by any other of the disciples of the Lord, and what things Aristion and the presbyter John, the disciples of the Lord, say. For I did not think that what was to be gotten from the books would profit me as much as what came from the living and abiding voice." (in Eusebius, Church History, 3:39:4)
Apparently, Papias was unaware of any of the apostles having disagreed about the resurrection or other major doctrines of the faith. He speaks of the apostles as if they were all faithful to one common message.
Remember, the unfaithfulness of Judas is mentioned by all of the gospels, Acts, Paul, Papias, etc. When men like Judas and Demas departed from the faith, their unfaithfulness is mentioned prominently. Even the lesser errors of the early church leaders, such as the unbelief of Jesus' brothers and Peter's denials of Christ, are mentioned by the earliest sources, so it seems unlikely that people like Paul and John had rejected the resurrection or any other major element of the faith without anybody mentioning it.
What was the nature of the early Christian belief in the resurrection? Did they merely think of it as a good concept accepted on philosophical grounds? No, the early Christian sources describe Jesus' resurrection as a historical event with many eyewitnesses, and being an eyewitness was a requirement for apostleship (Acts 1:21-22, 1 Corinthians 9:1). The early Christians were concerned with eyewitness testimony (Acts 2:22, 2:32, Hebrews 2:3, 1 Peter 5:1, 2 Peter 1:16, 1 John 1:1-3), not merely whether the resurrection and other purported miracles seemed philosophically appealing.
We don't have all of the details of early church history that we would like to have. We don't have detailed early accounts of how some of the apostles died, what sufferings all of the apostles went through prior to death, etc. But we do have a lot of evidence that the resurrection was considered one of the most important parts of the Christian faith and that the apostles and other early Christians suffered for that faith. We have early accounts of the martyrdom of some of the apostles, and later accounts of the martyrdom of other apostles are consistent with earlier, more general accounts of early Christian suffering.
What do skeptics want us to conclude? That somebody like Paul considered Christian faith "worthless" without Jesus' resurrection (1 Corinthians 15:17), yet he died for his faith without believing in the resurrection? Do they want to suggest that all of the early sources that refer to Paul suffering for his faith are unreliable, including Paul himself? If so, why? Are we supposed to conclude that men like Peter and John suffered for a faith that didn't include the resurrection? It seems that what these skeptics are trying to do is raise insignificant doubts in people's minds, even though the doubts aren't sufficient to overturn the Christian argument. The fact that no early sources refer to Thomas dying for the resurrection, for example, does nothing to overturn the fact that we have reliable evidence for other people suffering and dying for a faith that included the resurrection as one of its most significant elements, an element considered indispensable.