The problem, simply stated, how did the church function before canon was determined and wide spread?
From the beginning to today, it's always been the leading of the Spirit of God. The canon originated from the mind and Spirit of God, it is God's canon, his hand in history. Christ the head of the Church is the head of the canon. Without the head, what can the body do but nothing? Only walking by faith in the Spirit has the Church ever functioned. Through reading Paul's letters, it becomes clear the Church struggled to walk in the Spirit and not in the flesh from the earliest of New Testament times. However it is God, through the sovereign grace and mercy of His providence of the leading of the Spirit in God's people who faithfully preserved and maintained the oral and written Word of God throughout history.
The upper room, the Spirit of God raining down on his people is a key point and time for the New Testament Church.
As to before, this Psalm of David comes to mind...
Psalms 51
1 Have mercy on me, O God,
according to your steadfast love;
according to your abundant mercy
blot out my transgressions.
2 Wash me thoroughly from my iniquity,
and cleanse me from my sin!
3 For I know my transgressions,
and my sin is ever before me.
4 Against you, you only, have I sinned
and done what is evil in your sight,
so that you may be justified in your words
and blameless in your judgment.
5 Behold, I was brought forth in iniquity,
and in sin did my mother conceive me.
6 Behold, you delight in truth in the inward being,
and you teach me wisdom in the secret heart.
7 Purge me with hyssop, and I shall be clean;
wash me, and I shall be whiter than snow.
8 Let me hear joy and gladness;
let the bones that you have broken rejoice.
9 Hide your face from my sins,
and blot out all my iniquities.
10 Create in me a clean heart, O God,
and renew a right spirit within me.
11 Cast me not away from your presence,
and take not your Holy Spirit from me.
12 Restore to me the joy of your salvation,
and uphold me with a willing spirit.
ESV