The apostles' willingness to suffer and die for their proclamation of the resurrection is a significant line of evidence, not because martyrdom proves a belief is true, but because it proves the belief is sincere.
Many people have died for beliefs they held sincerely — but the apostles' situation is unique. They were not dying for a belief passed down to them; they were dying for something they claimed to have personally witnessed. If the resurrection had not occurred, the apostles would have known it was a lie.
Traditional accounts record the martyrdom of nearly all the apostles. Peter was crucified upside down in Rome under Nero. Paul was beheaded in Rome. James son of Zebedee was killed by Herod Agrippa I (Acts 12:2). James the brother of Jesus was thrown from the temple and clubbed to death according to Josephus and Hegesippus.
Even before martyrdom, the apostles endured years of persecution. Paul catalogs his sufferings in 2 Corinthians 11:24-27: five times receiving thirty-nine lashes, three times beaten with rods, once stoned, three times shipwrecked. No one endures this for decades for a known fabrication.
The transformation of the apostles from frightened deserters at the crucifixion to bold proclaimers within weeks requires an explanation.