Bro agedman,
Thank you for your questions. I find that I sometimes do not go into sufficient depth to clarify my position. When I am questioned, it helps me to be more specific regarding a certain point.
My point in our refusing to baptize someone who refused to become a member our church is aimed at someone who believes that baptism is simply optional. Christ did not leave it optional to his church but commanded them (Mt 28:19) to exercise that ordinance that only they had authority to administer. Since authority is given to the church (Lu 9:1), it no longer is in the hands nor does the decision belong to the individual. Not only is the command to baptize contained in Mt 28:18 but also to immerse in the doctrines of the new covenant which reveals the only true God (Jn 17:3). Since we believe that the Holy Spirit is given to the church, it is where we must abide in Christ in order to come to this knowledge and manifold knowledge (Eph 3:10) of his works for his people. Therefore, a person refusing baptism also refuses church covenant membership and denies church authority. We do not believe that someone who would take such a position has come to a sufficient knowledge of what salvation truly constitutes.
The Ethiopian treasurer was a proselyte as evidenced by the verse in Acts 8:27 saying, "...had come to Jerusalem for to worship." This verse proves he was not ignorant of the scriptures and was under the old covenant teaching. Philp went into great depth in teaching which resulted in the treasurer desiring baptism. It would seem that either he was joined to the church at Antioch or because of the caravan of people traveling with him, he and the people accompanying him, as Cornelius and his house, the Phillipian jailer and his house, or Lydia with her house, became a new body of Christ.
If a visitor came to Christ while with us and desired to be baptized but wanted membership at another church we would, presuming that church was of like faith and practice, baptize them and make them a member of our body. At a later date when they returned home, we would transfer their membership to that like-minded body by letter.
The question regarding Paul is even more difficult. It is difficult to believe that, given Paul's extensive travels, he would have gone many years without observing the Lord's supper before returning to Antioch. It is the practice of some Baptist churches to direct the evangelist to baptize the initial members of the new assembly into the parent church until they have sufficient members to continue on their own. If this were the case, Paul, as a member at Antioch, would have been authorized to observe the Lord's supper with the new members. After they became a new body, he would no longer be authorized to do so.
Regardless, we as a body of Christ at Monarch, do not practice open communion.
May the Lord lead us unto all truth.