The Catechism in Questions & Answers

15. How did the biblical books come to be collected?

The biblical writings were collected over the course of centuries. This not only came about as the result of human contemplations, but also especially through the will of God.
The Christian canon of the Old Testament is based on the Hebrew canon of Judaism, the writings of which are presumed to have come into being over a timespan of about 1,000 years.
The revelations of God The canon of the New Testament consists of the gospels, the Acts of the Apostles, the letters of the Apostles, and one prophetical book, namely the Revelation of Jesus Christ. The writings of Apostle Paul were the first to gain esteem in the early church. The gospels— of which the gospel according to Mark is the oldest— and the other writings were added later on. The writings of the New Testament came into being over the space of approximately 70 years.
In order to preserve and hand down these original accounts of the Christian faith, they were compiled into a collection that ultimately came to be certified as “canonical” in the course of various synods.

The collection of writings that are definitive for the teachings of a particular religion are called a ‘canon’. For the Christian faith, these include the writings of the Old and New Testaments.
The term ‘synod’ is derived from the Greek word synodos and means ‘gathering’ or ‘assembly’. A synod is understood as the assembly of an ecclesiastical body that has the authority to pass binding resolutions.