They gathered and choose the books of New Testament over span of several years. They organized and guided the early Church, they defined the dogmas of Faith (like Divinity of Christ, the unity of three Persons of Holy Trinity, unity of two natures of Christ in one person etc ...
When the last books of the New Testament were written the New Testament did not exist yet as one collection. The basic dogmas of Faith were formed generations later. This process took generations and was inspired by the Holy Spirit.
That was really little different than what I Corinthians 14 exhorts every local body to do in terms of vetting evidently Holy Spirit fostered pronouncements in the local group.
BTW, I don't really consider the church of that era the Roman one.
IIRC, Rome's usurping Bishop didn't weild enough political power to make that stick for some time to come.