Paul identifies three "pillars" of the early church James (Jesus' brother), John, Peter kjv@Galatians:2:9. Modern Catholics identify only one: Peter. It is James here that delivers the group's verdict. John is either silent or absent. Paul is portrayed once again as serving the church under their authority (even when he has disagreement). I have no doubt that the Spirit was sought for this momentous decision but is not quoted. There is plenty of OT text regarding the inclusion of gentiles, but not mandatory circumcision of them. The decision is based then upon the consistency of the doctrine of saving grace.
Another of the mysterious ways of God. That He could speak to one man in the group and not the others, that this would be His will as if challenging the one man to prove it to the rest/himself. This is not always the case but, seems to happen more times than not. Notice that He doesn't even tell that one man what all he must do, instead stringing together a sequence of other single men and revelations again as if challenging them/him. These wonders are often tests of faith/obedience rather than rewards of such.