Before Vatican II Catholic faith and practice were strong, rooted in the Gospels, their authenticity and sequence affirmed by Irenaeus. He was taught by Polycarp who was martyred rather than deny what he had learned about Christ from the apostles, in particular John. (Adv. Haer. III, I, 1). 

Since Vatican II, however, Catholic seminaries and universities have promoted an ahistorical "Markan Priority" theory that, at this writing, indefensibly deems it "untenable" that apostle Matthew wrote The Gospel according to Matthew and reduces all four Gospels to probable unwitnessed hearsay: usccb.org/bible/matthew/0, mark/0, luke/0, john/0, last paragraphs — uncertainty that may explain why few homilies now include the "hard sayings" of Jesus upon which salvation may well depend. 

The key claim of this theory is that because "You are Peter ..." (Mt 16:18-19) is not in Mark (8:27-30), Jesus never gave primacy to Peter. But Irenaeus wrote, "Mark, the disciple and interpreter of Peter, did also hand down to us in writing what had been preached by Peter." (Adv. Haer. III, I, 1). With Nero's agents listening, Peter would not have ended his ministry by proclaiming a "kingdom" to which he held the Keys.