The matter of canon was settled long ago by the blood, sweat, and tears of Holy men of God throughout the history of the Church beginning with St. Paul. Throw out the Old Testament and you throw out the prophecies of the coming Messiah, and nullify the teachings of Christ, even;
Matt 5:17 “Do not think that I came to destroy the Law or the Prophets. I did not come to destroy but to fulfill. 18 “For assuredly, I say to you, till heaven and earth pass away, one jot or one tittle will by no means pass from the law till all is fulfilled. 19 “Whoever therefore breaks one of the least of these commandments, and teaches men so, shall be called least in the kingdom of heaven; but whoever does and teaches them, he shall be called great in the kingdom of heaven."
The New Testament is also full of Old Testament quotations, and without it, the New would make no sense, neither to ancient nor modern readers.
Eutychus...
I feel it was something more than men with pen in hand. In fact, I place the authorship of the New Testament squarely in the hands of the Holy Spirit. For there are New Testament prophecies in the New Testament as well. Here are a couple...
"But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things TO YOUR REMEMBRANCE." John 14:26. The 4 gospels are the product of the memories of the authors.
"But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, HE SHALL TESTIFY OF ME..." John 15:26. The Holy Spirit 'testified' in these men, and they wrote the testimony thereof.
"Howbeit when he, the Spirit of truth, is come, he will guide you INTO ALL TRUTH: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: AND HE WILL SHEW YOU THINGS TO COME" John 16:13. The Holy Spirit guided the authors in life, and even caused them to shew us things to come, that is to say, in the future. Among those things, is the New Testament.
And one more, "The Revelation of Jesus Christ, which God gave unto him, to SHEW his servants things which must shortly COME TO PASS." Rev 1:1.
That is how the New Testament came to be...instead of speaking directly to prophets, as He did in the Old Testament, God sent the Holy Spirit to dwell within the spirits of the apostles and others. Their telling, in letters and to ministry members, their speaking of their experiences with the indwelling Holy Spirit, directly led to the New Testament.
Let's end with an Old Testament prophecy from Isaiah...that speaks directly to the New Testament.
"Seek ye out of the book of the LORD, and read: no one of these shall fail, none shall want her mate: for my mouth it hath commanded, and HIS SPIRIT HAS GATHERED THEM." Isa 34:16