Colossians: Day 19

Verses 4:10-15
My fellow prisoner Aristarchus sends you his greetings, as does Mark, the cousin of Barnabas. (You have received instructions about him; if he comes to you, welcome him.) Jesus, who is called Justus, also sends greetings. These are the only Jews among my co-workers for the kingdom of God, and they have proved a comfort to me. Epaphras, who is one of you and a servant of Christ Jesus, sends greetings. He is always wrestling in prayer for you, that you may stand firm in all the will of God, mature and fully assured. I vouch for him that he is working hard for you and for those at Laodicea and Hierapolis. Our dear friend Luke, the doctor, and Demas send greetings. Give my greetings to the brothers and sisters at Laodicea, and to Nympha and the church in her house.

Questions:
1. What does it mean to be a co-worker in there Kingdom of God?