So Saul the Persecutor of Christians shows up in Jerusalem claiming to be a changed man. "Yea right. My brother still has bruises from the beatings he received after you sic'd those bullies on him" Yes Christians believe in forgiveness and all that but nobody really wanted to get too close to Paul. Resurrection is one thing. Someone completely converting to the very movement he's been persecuting? Seems a little suspicious.
It was Barnabas who befriended Paul and took him around to all the disciples and made them act their faith and accept him.
And then later on it was Barnabas who stood up to Paul in defense of his nephew Mark. Mark for some reason had left Paul and Barnabas in the middle of a missionary journey and Paul was understandably reluctant to take Mark on another trip. But Barnabas insisted to the point that he took Mark on his own trip and let Paul and Silas go a different way.
Mark proved worthy of Barnabas' trust and later on even Paul had good things to say about him.
I know you have to be careful here when it comes to trusting people and giving them more chances to stab you in the back. Sometimes people really do use up their chances. But most of us could probably stand to give out a few more chances. Thank God we haven't used up our chances with God.