And he went up on the mountain and called to him those whom he desired, and they came to him. And he appointed twelve (whom he also named apostles) so that they might be with him and he might send them out to preach and have authority to cast out demons. He appointed the twelve: Simon (to whom he gave the name Peter); James the son of Zebedee and John the brother of James (to whom he gave the name Boanerges, that is, Sons of Thunder); Andrew, and Philip, and Bartholomew, and Matthew, and Thomas, and James the son of Alphaeus, and Thaddaeus, and Simon the Zealot, and Judas Iscariot, who betrayed him.
Jesus earthly ministry was marked by miracles and wonders, tremendous acts of kindness, love for others and deep teaching. Jesus knew his path would take him to the cross. He knew he would return to heaven for a time. He planned for his departure. He prepared his disciples to continue to spread his message after he returned to heaven.
These men were first hand witnesses to his ministry. They saw his miracles, observed his love for others, took in his teaching and all but one carried his message to others after his ascension back to heaven. These eleven were given the chance to withdraw their support for Jesus but ten of them died as martyrs refusing to refute who Jesus really was. Why would these men give their lives to a man who died?
The answer is clear. They knew the truth of who Jesus was and were not ashamed of it and were not afraid of the consequences. Jesus is the Son of God. He is the Saviour.