Many were martyred during the Roman persecutions of Catholics. Martyr is actually a Greek word for witness. These faithful Christians tried to avoid persecution, but if hours of torture and a horrible death resulted from witnessing to the faith, they accepted it.
St. Stephen was the first deacon of the Catholic Church and the first Christian martyr (also called the Proto-martyr). He was stoned to death for being a Christian (Acts 7:58). Saul of Tarsus was present at the event and later became St. Paul the Apostle. Emperor Nero had him beheaded. (Because St. Paul was a Roman citizen, unlike St. Peter and the other apostles, Paul couldn’t be crucified, but he could be beheaded.) Peter was crucified upside down at his own request, because he felt unworthy to die in the same manner as Jesus.
All the apostles were martyred except St. John, known as the beloved disciple. His persecutors tried to burn him alive in boiling oil, but he survived. Exiled to the island of Patmos, he wrote the Book of Revelation. He died of old age in a.d. 100. For more on the martyrs, check out the stories of St. Agnes, St. Agatha, St. Lucy, St. Sebastian, St. Lawrence, and St. Cecilia. These saints endured swords, arrows, being burned alive, and more in the name of their faith.