What proves a religious document true? What makes it so that those books that are in the Catholic Bible but not Protestant Bibles are or aren’t scripture? How do you know if a manuscript is God-Breathed, and meant to be part of the Bible?
Thanks for asking this really important question.
What we call the Old Testament is the collection of Hebrew Scriptures that Jesus would have been familiar with and he often referred to it with phrases like ‘The Torah and the Prophets’, ‘the law, the prophets, and the writings’, or ‘Moses and David’. This collection is also referred to today as the TaNaK - Torah (Pentateuch), Nevi’im (Prophets) and Ketuvim (writings). This collection of scrolls developed from a multi step process including oral traditions and various written traditions. By Jesus’ time, this collection was firmly established.
Your question however rightly questions what happened after that. Why did some books make the New Testament, while others made the apocrypha which isn’t considered canon (divinely inspired) by all Christians.
Wesley Huff, a Bible scholar has written a helpful article The Question of Canon: the who, what, and where on the Books of the Bible (attached fully below) but I wanted to share his main points as a response to your question.
Firstly, why did the Old Testament need to be added to at all? The Hebrew scriptures was seen by many as incomplete due to the Jewish belief in the awaited messiah. This meant that the scriptures were poised ready to be completed by some conclusion of some sort.
The anticipation for the deliverance of Israel discussed in the Old Testament (OT) was still very much in the sub-conscious of first century Jews.2 We see this language of deliverance and redemption in the biblical testimony in places like John 1:41 (looking for the Messiah), Luke 2:38 (redemption of Jerusalem), Luke 2:25 (consolation of Israel), and Acts 1:6 (restoration of the kingdom). In other words, the Jews within early Christianity did not view the story of the OT as complete. As N.T. Wright states, “the great story of the Hebrew scriptures was therefore inevitably read in the second-temple period as a story in search of a conclusion.”
Secondly, the theological themes and focuses of divinely inspired NT manuscripts fit those of the Hebrew Scriptures. This means that extraneous gospels like the Gospel of Thomas can be identified as not canonical because it doesn’t hold to the culmination of this theme in the same way. Huff writes about the theme of covenant as smoothly threading through the Hebrew and canonical Greek texts.
Equally, first century Judaism as well as early Christianity was covenantal. When looking at the writings leading up to the first century, an interpretation of what can be described as covenantal categories is seen very strongly. The Jewish people understood the actions of God through the lens of His covenantal promises. What is interesting is that when we get to the earliest Christian writings this theological covenantal mind frame crosses over.7The acts of Jesus circulating in the Kerygma (oral Jesus stories) and recorded in the gospels were framed in the context of Jesus bringing with Him a new covenant. The last supper was understood as a covenantal meal, as Jesus describes (Luke 22:20; Matt. 26:28; Mark 12:4); Zachariah understood the coming of his son, John the Baptist, as the fulfilling of God’s covenant and the coming Messiah (Luke 1:5-25); Paul describes the ministry of the Apostles as being “ministers of a new covenant” (2 Cor. 3:6), alluding to the “making [of a] new covenant with the people” in Jer. 31:31; and so on. There are many examples of this in the text.
Thirdly, In deciding which manuscripts were considered divinely inspired and trustworthy, there were several criteria, including proximity in time to the events (many believe Mark to be written within 20 years of Jesus’ death and possibly the earliest account). 1 Cor 15 records possibly the earliest hymn or creed used by the early Christians. There is a consensus (with some disagreement) that by the end of the 2nd century AD at the latest, the full canon that we have today was formed, based on writings by people like Iranaeus. His writings include assumption that other Christians already well knew the texts to which he referred. Contemporaries of Iranaeus, Theophilus bishop of Antioch, and Clement of Alexandria similarly included such references in their writings.
Along with the four-fold gospels, he names the entire Pauline corpus (excluding Philemon), Acts, Peter, James, Hebrew, 1st and 2nd John, 1st and 2nd Peter, and Revelation. Irenaeus quotes BNTC passages over a thousand times, identifying them directly as “graphe ” or scripture (there is also some evidence he thought the Shepherd of Hermes was scripture but that will be discussed later). However, because of this fact many scholars have posited the idea that Irenaeus was the innovator of the BNTC, Elaine Pagels describes him the “principal architect”10 of the BNTC itself. The problem with such a theory is that in Irenaeus’ Against Haresies he gives no proof-texting for his citations of BNTC being scripture. Instead he cites the BNTC frequently, as if his audience would have known of them, and confidently as scripture.
Fourthly, even where apocryphal books were quoted by early church fathers, there’s an indication that they were not seen as canonical, due to the differing content and context of these writings to the canon.
.15 Eusebius (260-339 CE) refers to the apocryphal Gospel of Peter when the Christian community of Rhossus writes to Serapion of Antioch (191-211 CE) in regard to the supposed work of Peter. Serapion, after reading the Gospel of Peter discovered it contained a docetic teaching (that is, denying the incarnate physical nature of Christ). Serapion condemned the apocryphal gospel of Peter on all counts because of its content. 16 In regard to Serapion and the community of Rhossus, can we draw the conclusion (as previously stated) that the four-fold gospel collection that we find in the BNTC was actually normative? The answer remains that there seems to be no indication on the part of the Serapion event, that the Gospel of Peter was either considered as scripture by the church at Rhossus or Serapion himself.
Interestingly, the Epistle of Barnabus and the Shepherd of Hermas writings was one of the contenders that could have been included in the canon. Huff explains why ultimately they were not included in canon:
The issue with the Shepherd as a book was its late authorship; being a mid to late second century writing, with no direct apostolic connection. It is interesting to note that the Muatorian fragment (mentioned previously) rejects the Shepherd (along with the Epistle of Barnabas) outright stating that “it was written very recently, in our times by Hermes while his brother Pius was sitting in the chair of Rome."20 This gives a very strong indication that recent productions (after the first century) were never truly seen as contenders to BNTC. Apostolic succession for documents of the BNTC was considered the core criteria for the early Christian church and its leaders. All in consideration, while Irenaeus seems to have no interest in Barnabas, he does at one point call the Shepherd of Hermes graphe , “scripture” in his work Against Heresy , but afterwards never mentions it again. Origin too regarded the Shepherd as valuable but leaves it out in his explicit list of BNTC books. Tertullian, much like Irenaeus had very little to say about Barnabas but rejected the Shepherd outright stating, “But I would yield my ground to you, if the writing of the Shepherd… had deserved to find a place in the Divine canon; if it had not been habitually judged by every council of Churches (even your own) among apocryphal and false (writings) 21”. The Shepherd of Hermes remains the closest contender for inclusion in the BNTC, but as seen, the leaders of the church did not seriously consider it part of canon in the second century.
There has been some interesting discussion on the Shepherd of Hermas elsewhere here in case you’re interested:
It’s a vastly fascinating topic which I’m sure others can expand on. I hope this is a helpful starter with considering the issues though.