Martin Luther, as summarized and quoted by Jaroslav Pelikan, in Divine Rhetoric (pp. 90), from Luther’s own Introduction to his New Testament:
I ask yet again: Is 1 Corinthians 15 the gospel? Is the gospel found in the sermons in the Acts? Why did they call the first four books of the New Testament “The Gospel”? All of this is discussed in The King Jesus Gospel.
“John’s Gospel and St Paul’s Epistles, especially that to the Romans, and St Peter’s first Epistle are the true kernel and marrow of all the books.” A few sentences later he [Luther] added Galatians and Ephesians to Romans, but he did not add the synoptic Gospels to St John. For, as he explained, “John’s Gospel is the one, fine, true, and chief Gospel, and is far, far to be preferred over the other three and place high above them. So, too, the Epistles of St Paul and St Peter far surpass the other three Gospels, Matthew, Mark, and Luke.“