We believe in the 39 books of the Old Testament, because the Lord Jesus Christ affirmed the Old Testament. And we believe in the 27 books of the New Testament, because the Lord Jesus Christ authorized His apostles to write the New Testament.
The doctrine of canonicity ultimately comes back to the lordship of Jesus Christ. If we believe in Him and submit to His authority, then we will simultaneously believe in and submit to His Word. Because He affirmed the Old Testament canon, we also affirm it. Because He authorized His apostles to write the New Testament, we likewise embrace it as well.
It was not the Catholic church that determined the canon. Constantine did not determine the canon. Joseph Smith certainly did not determine the canon. No, it is the authority of Christ Himself, the Lord of the church and the incarnate Son of God, on which the canon of Scripture rests.
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