The deaths that the apostles died gave authenticity to the gospel. It was one of the reasons for the explosive growth of the church in the decades following the death and resurrection of Christ. There were probably a million Christians within decades.

Not only did the Apostles die gruesome deaths, Jesus gave them the power to perform miracles. The same kinds of miracles that Jesus performed. They could instantly raise the dead, heal the sick, give sight to the blind, make the deaf hear and the dumb to speak.

Paul said this:

(1 Corinthians 15:3-8 NIV) For what I received I passed on to you as of first importance : that Christ died for our sins according to the Scriptures, {4} that he was buried, that he was raised on the third day according to the Scriptures, {5} and that he appeared to Peter, and then to the Twelve. {6} After that, he appeared to more than five hundred of the brothers at the same time, most of whom are still living, though some have fallen asleep. {7} Then he appeared to James, then to all the apostles, {8} and last of all he appeared to me also, as to one abnormally born.

The church grew exponentially at a time when hostile skeptics would have exposed Christianity as false if it was. But Christianity is true. There must have been a huge number of people who either saw a miracle or talked to those who had.