Even other Christians were unsure of Paul because of the fierceness of his former persecutions. Barnabas' testimony about Paul's conversion and boldness convinced the apostles, because they trusted Barnabas. Through Barnabas, Paul came into fellowship with the apostles and became one of them himself. He entered into God's place for Him through the power of God-ordained relationships. Unless we go further, we have lost the invitation that Jesus and the apostles gave.
Unbelievers often get the wrong invitation.
To be sure, the Church will be a great gathering. It will not be a small insignificant group. But it will also be an intimate group. A mentor is someone who builds a personal relationship with a young believer, has become a role model, and is giving guidance to bring the new believer to maturity. Hopefully the one being mentored will also eventually become a mentor.