The Old Testament Scriptures record the process of collecting its books. From the beginning of the written Law, Moses gave the Law to the priests to care for it (Deut 31:9). God commanded that the Book of the Law be placed in the Ark of the Covenant for which the Levites oversaw (Deut 31:9, 24–26). From Moses on, books were added to the Law as they were written by inspired men. The book of Joshua recorded that Joshua wrote in “the Book of the Law of God” and added his inspired words to the Law of God (Josh 24:26). Even unto the writing of Chronicles, the Book of Kings was written into the Law of the Lord (2 Chr 35:26–27), and the Book of Kings reads that the Chronicles were added to the Law of God (2 Kings 23:28).
The Old Testament also records that Israel preserved the Law through the centuries. Even until the reign of Jehoshaphat, the Law was being taught in all the cities by the prophets and by the Levites (2 Chr 17:9). The Law was found in the keep of the Levites and read before King Josiah and all Israel before Israel’s captivity (2 Chr 34:17–21; 2 Kings 22:8–16; 23:2–3, 21–28). Then even further to the time of Israel’s restoration, Ezra is described as a priest and the scribe of the Law of God (Ezra 7:11–12), and he returned from Babylonian captivity and set his heart on the study of the Law of the Lord (Ezra 7:10). The Book of Nehemiah also recorded Ezra as being the scribe of the Law and reading it to the remnant who had returned from captivity (Neh 8:1). The Law was kept and copied throughout the centuries from Moses to Ezra, which is over a millennium. Prejudice would discount the reliability of the scriptures affirmed by the Septuagint, the Dead Sea Scrolls, and more.
Unto the first century AD, the New Testament writings confirm the existence of the written Law of Moses. For which, there was no debate over what Scriptures were inspired. Jesus confirmed the Old Testament collection by mentioning its three divisions excluding the addition of the Apocrypha, which was added 2000 years after the completion of the Old Testament. (Apocrypha means “hidden away.”) Jesus taught, “These are my words that I spoke to you while I was still with you, that everything written about Me in the Law of Moses and the Prophets and the Psalms must be fulfilled” (Luke 24:44). Jesus was clearly using all of the Scriptures for He also said, “And beginning with Moses and all the Prophets, He [Jesus] interpreted to them in all the Scriptures the things concerning Himself” (Luke 24:27). Jesus had no doubt that God oversaw and protected the Scriptures. Lastly, as a scholarly man explained regarding the collection of the books of the Bible, “If you prove the Bible, then you prove the collection of the Bible.” He was saying that the numerous evidences proving that the Bible came from God proves to us that God oversaw and keeps His Book from a state of distortion and corruption.
See also “How the New Testament Came Together.”