It is interesting what you come across in Scripture if you attempt to just look at what it says and think about how it would have been received originally.
With Onesimus, a faithful and beloved brother, who is one of you. They shall make known unto you all things which are done here.
Onesimus had not been a brother when the people in Colosse knew him before. But now, because he is a Christian, he is a brother and considered to be “one of” them. He had not been baptized into their membership. He had not even been saved when he lived there before, but already he is one of them.
Maybe New Testament churches had a slightly looser idea of church membership than some of us do.