New Testament the Anchor Bible Dictionary), may be the earliest known New Testament canon attributed to mainstream Christianity. It is similar, but not identical, to the modern New Testament canon. The oldest clear endorsement of Matthew, Mark, Luke, and John being the only legitimate gospels was written "circa" 180 AD. A four gospel canon (the "Tetramorph") was asserted by Irenaeus, who refers to it directly in his polemic "Against the Heresies", "It is not possible that the gospels can be either more or fewer in number than they are. For, since there are four zones of the world in which we live,
Synoptic Gospels that have been fulfilled among us, just as they were handed on to us by those who from the beginning were eyewitnesses and servants of the word, I too decided, after investigating everything carefully from the very first, to write an orderly account for you, most excellent Theophilus, so that you may know the truth concerning the things about which you have been instructed. The "synoptic problem" is the question of the specific literary relationship among the three synoptic gospels—that is, the question as to the source or sources upon which each synoptic gospel depended when it was written. The