How do we know 10 of the disciples were martyred?
How do we know that every apostle except John died for his faith, and are those sources reliable?
The tradition of apostles' martyrdom goes back at least to the beginning of the third century. In his third commentary on Genesis, Origen of Alexandria (ca. 185-254) writes that the apostles divided up the work of evangelizing the world between them—Peter, for example, took Pontus, Galatia, Cappadocia, and Bithynia, and at "the last came to Rome, and was crucified head-downwards; for he requested that he might suffer thus." According to Origen, other apostles went elsewhere; Thomas was assigned Parthia (today's India), and John was given "Asia."
Scholars debate as to where Origen picked up his information—some argue that he drew from the roughly contemporary Acts of Thomas, an apocryphal book relating Thomas' adventures as a missionary in India. That book states that Jesus' original 11 disciples "divided the countries among them, in order that each one of them might preach in the region which fell to him and in the place to which his Lord sent him."
But there are other sources to consider as well. Eusebius (ca 260-341) wrote perhaps the most complete history of the apostles, though he merely quoted other bishops for his authority. Acts 12: 2 tells us, for example, that Herod Agrippa had James, the brother of John, executed. To this, Eusebius adds the story told by the bishop Clement of Alexandria (d. ca. 215)—Origen's mentor—that "the person who led James to the judgment-seat was moved when he saw him bear witness, and confessed that he himself was also a Christian."
Or take the death of Philip, which bishop Polycrates of Ephesus (130-196)—again ...