Since Barnabas and Mark are Kingdom saints, why did they minister with Paul?

Just started listening to y’all recently, I am loving y’all’s teachings and preaching. I was wandering why Barnabas and Mark went with Paul helping, I thought they are kingdom saints.

Yes, Barnabas and Mark as well as Silas were Kingdom Saints.  However each one came to understand that their hope had been postponed and that God had called Paul to the ministry of grace to the Gentiles.  So if God’s ministry to Israel ended and he turned to the Gentiles, they joined Paul in helping him in his ministry to the Gentiles – Acts 9:27;  Acts 13:2; Acts 15:25-27, 32-40.