Thank you for the question on the chronology of the Bible, “In The Bible, Genesis is the first book of the Old Testament, but what is the first book of the New Testament?”
As one reads the table of content of the Bible, one reads that Genesis is the first book of the Old Testament and Gospel of Matthew is the first book of the New Testament.
As the scrolls were collected, teachers of the Law of the Old Testament ordered them just as the early church fathers ordered the “New Testament” books. The exact reason for the order of the Gospels is unclear. It may well be that Matthew was placed first since it was written to the Jews and Mark to the Gentiles. Luke was made part of the Gospels even though it was written to one man. The chronology of the Law is unique from Matthew and Mark. John being the last Gospel was likely to the Christians to affirm the deity of Jesus Christ as the God-Man who became flesh to bring God’s salvation to mankind.
If John was made place first instead of Matthew, it doesn’t affect the content of the Gospels. Each book is unique in itself. Historically, it has been accepted by the early church that Matthew is assigned the first book of the New Testament.
It may make interesting reading to make Mark first for its brief synopsis, followed by Luke for its chronology of the life of Jesus,, Matthew for the teaching of Jesus,, and then John for the personhood of Jesus Christ.
Summary: Matthew may have placed first in the New Testament to address the Jews view of who Jesus Christ is. It contains many of Jesus’ teaching.
-Kingston