At the same time, God has told us He communicates His will to us and for us by His holy word, made comprehensible by the Holy Spirit.
Whatever the Apostles may or may not have said before they wrote the Scriptures does not matter in the least. It's all conjecture. What we know is true is the inspired, written word of God.
And for those who have been graced with faith, that is more than enough.
"And take the helmet of salvation, and the sword of the Spirit, which is the word of God" -- Ephesians 6:17
It matters if you choose to question whether the Sacred Deposit of Faith preceded the Scripture, which is but an expression of it. Nor is it conjectural: the writings of the Church fathers who discussed the formation of the New Testament Canon are available to us.