Q. Which Gospel was written first?
A. The matter is complicated; scholars base their theories about which Gospel is prior on certain features of the four texts that can be interpreted in contradicting ways. Nearly all biblical scholars agree that Luke was not written first, and that John was written last, but the chief argument is whether Matthew or Mark came first.
The “Matthew first” position dates back at least to St. Augustine in the fourth century; it was generally accepted by Catholics until the mid-twentieth century. This hypothesis still has some support, in part because it carries the weight of ancient tradition and it follows the order in which the Gospels appear in the canon. The opposing hypothesis is that Mark was written first.