Along with uniting under a bishop and developing creeds, the Church came together to establish the official New Testament. Marcion tried to develop an anti-Semitic canon, and the people needed to officially establish which books were and were not legit.
They chose books based on "marks of apostolicity" (Cairns 118). Were they written by an apostle or a close associate?
Also, it had to edify when read out loud. Many people disputed authorship of some of the books, but ultimately, the Holy Spirit led them to place the 27 books of the New Testament in the canon as they are today. Jesus Christ established the Word of God for the Church. A committee only organized it.