A Christian martyr is a person who is killed for following Jesus. In early church years, this often occurred through stoning, crucifixion, burning at the stake or other forms of torture and capital punishment. The word "martyr" comes from the Greek word μάρτυς, mártys, which means "witness."
At first, the term applied to Apostles. Once Christians started to undergo persecution, the term came to be applied to those who suffered hardships for their faith. Finally, it was restricted to those who had been killed for their faith. The early Christian period before Constantine I was the "Age of martyrs". Early Christians venerated martyrs as powerful intercessors, and their utterances were treasured as inspired by the Holy Spirit."
The Greek word "martus" signifies a "witness" who testifies to a fact he has knowledge about from personal observation. It is in this sense that the term first appears in Christian literature; the Apostles were "witnesses" of all that they had observed in the public life of Christ. In Acts 1:22, Peter, in his address to the Apostles and disciples relative to the election of a successor to Judas, employs the term with this meaning: "Wherefore, of these men who have accompanied with us all the time that the Lord Jesus came in and went out among us, beginning from the baptism of John until the day he was taken up from us, one of these must be made witness with us of his resurrection".
The Apostles, from the beginning, faced grave dangers, until eventually almost all suffered death for their convictions. Thus, within the lifetime of the Apostles, the term martus came to be used in the sense of a witness who at any time might be called upon to deny what he testified to, under penalty of death. From this stage the transition was easy to the ordinary meaning of the term, as used ever since in Christian literature: a martyr, or witness of Christ, is a person who suffers death rather than deny his faith. St. John, at the end of the first century, employs the word with this meaning.