Albert Barnes on Acts 15:6

And the apostles and elders, etc. They came together in accordance with the authority in Mt 18:19,20. It would seem, also, that the whole church was convened on this occasion; and that they concurred, at least, in the judgment expressed in this case. See Acts 15:12,22,23.

For to consider this matter. Not to decide it arbitrarily, or even by authority, without deliberation; but to compare their views, and to express the result of the whole to the church at Antioch. It was a grave and difficult question, deeply affecting the entire constitution of the Christian church, and they therefore solemnly engaged in deliberation on the subject.

(f) "together for to consider" Mt 18:20