"When he came to Jerusalem, he was trying to associate with the disciples; but they were all afraid of him, not believing that he was a disciple. But Barnabas took hold of him and brought him to the apostles and described to them how he had seen the Lord on the road, and that He had talked to him, and how at Damascus he had spoken out boldly in the name of Jesus.
Before Saul of Tarsus' (Paul) conversion, he persecuted the Christians. He was instrumental in the death of many believers; even holding Stephen's cloak while the Jews stoned him to death. For that reason, it's no wonder the disciples were afraid of him and skeptical of joining with him. But God laid it on Barnabas' heart to stand up for Paul and prove to the disciples that Paul was authentic. Paul had "spoken out boldly in the name of Jesus." When we speak boldly of Jesus, there can be little room for doubt of our relationship with Him.