Our parish priest asked us at Mass which book of the New Testament was the oldest. Most of us said, Matthew. He said we were wrong, and that First Thessalonians was the oldest. This doesn't seem right does it?

When we speak of the age of a particular Book in the Bible we can speak about its age in terms of the events it describes, or of the likely date it was put into the written form we have today. Usually when scholars speak of the age of a Book they refer to the time of its appearance in final written form. And in this sense, it is largely accepted that the First Letter to the Thessalonians is probably the oldest book, the first of St. Paul’s letters, and written between 51‐52 AD. 
 
The writing of a letter in the New Testament was a fairly straight‐forward process and, while St. Paul and others may have had made some final edits, or even a second draft, it is likely he dictated it to a scribe, who wrote it and then had it sent within a matter of days. Other copies may also have been made and circulated. 
 
The emergence of the Gospels in written form was a much more complicated process. And while the events they detail are older (from the early 30s AD), the writing out of these events went through several stages. 
 
Obviously the first stage of the Gospels was the actual events themselves, the words and deeds of Jesus. But it will be noted that Jesus did not write a book, or even say to the Apostles, “Go write a book.” Rather, he sent them to preach, teach, and baptize disciples into the life of his Body, the Church. 
 
Thus, the second stage was the oral stage wherein the Apostles went forth proclaiming what Jesus taught and did and who Jesus is. During this time the teachings began to be written by scribes, collected and circulated. 
 
And thus, we begin to see the written stage. The idea that Matthew or John just sat down and wrote the gospel is probably inaccurate. Recall that most people could not write in the ancient world. Scribes and others acted as secretaries for the author who helped refine and edit the final product. Some think that Mark was Peter’s assistant and scribe. 
 
Gradually the gospels were collected and edited in what came to be their final form, as we know them. The exact dates and order of their final form are hotly debated topics among scholars. However, it is safe to say that the four gospels took their final form between 60 and 90 AD, some time after St. Paul had sent his letters.