Hi, friend. Thanks for the reply.
I am peer reviewing my reading/interpretation of this verse. To my mind it relates to a situation such as the one in Acts 15:
1 Â¶ And certain having come down from Judea, were teaching the brethrenâ€”â€˜If ye be not circumcised after the custom of Moses, ye are not able to be saved;â€™
2 there having been, therefore, not a little dissension and disputation to Paul and Barnabas with them, they arranged for Paul and Barnabas, and certain others of them, to go up unto the apostles and elders to Jerusalem about this question,
These would have come "from James" but, not tracking with James, they were not "of James." We might have the same thing here:
Ga 2:12 For before that certain came from James, he did eat with the Gentiles: but when they were come, he withdrew and separated himself, fearing them which were of the circumcision.
So, in my thinking, the pluperfect pegs the time as being relevant to when these self-appointed missionaries went out and spoke as if they were representing the community. So I get a reading like this:
19 They went originated from us [our community], but they were not [part] of us; for if they had been [part] of us, they would no doubt have been continuing [in line] with us: but they went out, that they might be made manifest that they were not all [representative] of us.