Who was the first full-time co-worker...

Here's a fascinating perspective on service. It's from a book by Witness Lee, The History of God in His Union With Man. Have a look:

Have you ever realized that the first full-time co-worker was Abel? He raised up sacrifices for God's offering. This was his job, his work. This was also his kind of living. Abel was a person doing nothing on this earth but offering to God. Genesis 4:4 says that God had respect unto Abel and his offering. How good it was that Abel was a full-timer doing nothing but living for God! He was not in the line of good and evil. He was in the line of the tree of life, living for God and living to God.

Those of us who are serving the Lord full time should compare our full-time service with Abel's. When we compare ourselves with Abel, we probably need to say that we are part-timers, not full-timers. Abel was living a life that was absolutely for God. His living was for God, not for food, for eating. His elder brother Cain was the opposite. Cain was living for his daily necessity, for food. 

Abel lived on God in a spiritual sense, and he lived on what God did, on what God provided, in a physical sense. If we are full-timers we should live as Abel did. We should not live on anything worked out by ourselves. Cain was different from Abel because he lived on what he did. He was a farmer producing food for himself. Thus, we can see that it is not too much to say that Abel lived a Christian life. If we live the Christian life merely according to what the New Testament teaches, that is not adequate. We have to come back to the Old Testament to see the first typical full-timer for God—Abel.