The Church decided the canon. How do you know you have the correct canon?
Someone may say, Does it really matter at the end of the day?
Aren't they just varying quantities of the same substance originating from the same source, that is, the mouth of God?
Every member has a different experience and testimony of how God has been changing them into the image of Christ.
Even though it is the same God and Father, the same Christ and the same indwelling Spirit.
All divine truth comes from the same source, out of the same mouth, the mouth of God.
We all get only fragments of truth. Some are personal revelations while some are for sharing with others.
Most of these fragments were never recorded. But I say that's OK too. As we have the same Spirit Who inspired those in the past. This same Person within us moves us along and leads us into all truth. Jesus Himself promised that.