Irenaeus of Lyons (130-202) stated that there must be 4 gospels and only 4 because there were 4 corners of the earth.  Thus, the Church should have 4 pillars.  This indicated that as early as 200, there existed a set of Christian writings somewhat similar to what is now the 27 books of the New Testament, including the 4 gospels.  However, most of our oldest manuscripts of the New Testament come from the 4^th century when Christianity was the recognized religion of the Roman Empire.