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God Himself provides us His Word.

He gave us the Apostles, who with their spiritual gift from God the Holy Spirit, glorifying the Son, and given the gift as the Father had planned, provided us those books.

Upon the completion of the canon in 96AD, those gifts ceased.

A believer cannot know the Word of God outside of the doctrine of faith in his spirit and soul. Doctrine (faith) comes by hearing and hearing by the Word of God.

We know from the admonition in Revelation of the completion of the Canon. John was the Last Apostle.

We know from Timothy of the adequacy and sufficiency of the Word for every good work.
2Ti 3:16-17
(16) All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:
(17) That the man of God may be perfect, throughly furnished unto all good works.

We know His work in us from the Holy Spirit teaches us the truth.
1Jn 2:25-27
(25) And this is the promise that he hath promised us, even eternal life.
(26) These things have I written unto you concerning them that seduce you.
(27) But the anointing which ye have received of him abideth in you, and ye need not that any man teach you: but as the same anointing teacheth you of all things, and is truth, and is no lie, and even as it hath taught you, ye shall abide in him.

We also know the doctrine of the Hypostatic Union, by which our Lord Christ Jesus provided us the example of the Christian way of thinking in this Church Age. Remaining in fellowship with God, through faith in Christ, never ceasing in prayer to the Father, and allowing God the Holy Spirit to perform His work in us to glorify the head, our Lord and Savior Christ Jesus.

How do you know that Appolinarius wasn’t right after all or Nestorius, or Eutyches for that matter?

Does Jesus have One Nature or Two Natures?

If the canon was completed c. 100 A.D. then why did it take 500 years for Revelation to be universally accepted by all Orthodox Christians?