The Church came into existence on the day of the Pentecost. We are all in this Church of the pentecost -- you might even say all in the pentecostal Church :-P
Constantine, for all of that talk about him "creating" the hierarchy was hardly that theologically capable. His tendencies in fact were towards Arianism. The Church existed before Him and in fact the persecutions were stopped first by Constantine's partner Augustus in the West
If one says that The Church came into existence in the 4th century then how come Pope Sylvester was already at hand to approve the Council from the point of the view of the Western Patriarchate? And how come the Bishop of Rome, Sylvester's predecessor, Pope Miltiades was presented with the Lateran residence by Constantine after he came to power -- Miltiades was already Pope at that time (being Pope from 310, two years before Constantine defeated Maxentius? And how come Miltiades' predecessor, Eusebius was banished by Constantine's predecessor Emperor Maxentius in 309? We can go further back to seeing that there was both a bishop in Rome (and of course bishops in Antioch, Alexandria, etc) and Christians writing against heretics
Finally, the canon was not "well known" in its entirety before "any Council said anything" -- note "in its entirety" -- there were canons created like Marcions which just included 10 Pauline Epistles, the Gospel of Luke (rather an edited version called the Gospel of Marcion) in 140 AD -- but you and I wouldn't accept that canon today
The OT was/is taken from the Septuagint
Though the Pauline Epistles were circulating freely, not all were considered scripture by all. The Gospels on the other hand were pretty unanimously accepted.
The next "canon" was Origen's which had all the books which we have now except for James, 2 Peter, 2 John and 3 John and it had the Shepherd of Hermas as scripture.
In fact in the first few centuries there were disputes about the letter to the Hebrews as well
you can hold that +Athanasius' canon was the most accurate but that also included the book of Baruch and the Letter of Jeremiah and left out the book of Esther (367 AD). In short, one cannot say the Canon in its entirety was well known before any Council said anything
Churches were originally in homes, not buildings.
There is no Biblical justification for the organized churches in denominations as seen today.
What Constantine did was unite the church with the State and thus, corrupt the church.
The OT Canon was established by Christ in the Gospels, without the Apocrypha (Law, Prophets and Writings).
The NT was accepted by the NT churches within the 2nd century as the books were circulated.
The Councils only recognized what the churches had long accepted as being part of the Canon.
The Apocrypha books were never officially considered as part of the Canon until Trent.
Catholics were allowed to disagree on their status in the Canon.