The Old Testament existed in the Torah [Jewish Scriptures]. They were inherited by the Early Church.
The New Testament came together piecemeal. It started out as a collection of writings. The earliest of which were probably St Paul who wrote within 37-years of the Crucifiction. There were more texts written but the Church under the guidence of the Holy Spirit, discounted all gospels except those known as the Canon [Matthew, Mark, Luke and John]. John was the last to write his. His is a more philosphical account which reflects his lifetime he spent reflecting before he wrote it. Mark on the other hand is a gospel written sooner to the crucifiction and shows one written in a hurry.
The letter of 1 Peter is credited as written by him. 2 Peter was believed not to be. Linus was Peter’s Scribe. [he was also commissioned by Peter and was later to become Pope St Linus. He may be the author of 2 Peter.
Paul’s letters were obviously his writings to the early communities of Christ’s Church. The same for the other letters [John,. James, Jude etc]. These were therefore not written as 'accounts of the Lord, but rather, to encourage the early Church. Had they been around today, they would no doubt have sent an email!]
Some writings were also called ‘Gospels’ but were either too long after the Apostles to be regarded as ahthentic, some were written for catechitcal purposes [and therefore not intended to be used as Gospels]. The text [sometimes called the Gospel of St Thomas] is an example of this. These 'documents were excluded from the New Testament as the Church only included those which were known to be authentic.
In this way, we KNOW that the contents of the New Testament are true.
Some Lutherans do not accept [or at least question] the Letter of James. There ar theological reasons for this. Luther excluded 7-books from his Bible which are found in the Catholic Bible. Some say it was because those books did not support his theories, so he kept them out to support his arguements. I do not know if that is the case. Some Lutheran Churches are now including the ‘missing’ books, as it is felt the Bible is imcomplete without them.
Luther was a Catholic Priest. He did not intend to start a ‘new’ church. He was an Augustinian Friar [the oldest surviving Religious Order in the Catholic Church]. If you ever get the chance to go to Clare Priory in Suffolk, it is wonderful to walk among the Cloisters in the dead of night [not at all spooky, very peaceful], over 1000-years old]…But, I digress. Luther after he was expelled for heresy, to prevent him starving to death [as the new Prods were not willing to support him], he was fed, clothed and sheltered by the Augustinians! I have no idea if he kept his vows of poverty and chastity.
His Bible also excluded books from the Old Testamant.
The oldest surviving parchments on which the New Testament was written [which have survived] are locked away in the Vatican.
Hoe that helps. Also go on line and see Catholic Encyclopedias.