One of the most certain facts about Jesus is He died.

Acts 4 Peter and John are threatened and arrested.
Acts 5 Apostles are pout in jail and flogged.
Acts 12 James the brother of John is put to death, and Peter is put in jail again.
2 Cor. 11.24-27 Numerous times Paul was almost put to death (Christianity is a great life)
Joseph reports James the brother of Jesus was stoned to death as a law breaker
Clement of Rome reminds us to reflect on the sufferings and martyrdom of Peter and Paul

Therefore, the Apostles truly believed what they were saying.