I listened to a podcast from Wayne Grudem about the cannon of scripture.
In 1. Timothy 5:18 Paul quotes Deutoronony but also Luke! "the laborer deserves his wages" is from Luke 10:7. In Acts, it is stated that Luke traveled with Paul. Now, liberal scholars could of course express that "Luke" is quite an ordinary. I find it highly unlikely that they are disjunct. What this means is, Paul read Luke's gospel and quoted it and thus giving it the same authority like scripture.
Same with Mark and Peter. Peter says in 1. Peter 5:13: "She who is in Babylon, chosen together with you, sends you her greetings, and so does my son Mark." Which Mark is he talking about? It is the same Mark that wrote the Gospel of Mark.
So the Gospel of Mark and Luke are both endorsed by the apostles Peter and Paul.
John and Matthew were both apostles.
Another interesting point: Peter in 2Peter 3:16 says "His letters contain some things that are hard to understand, which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction."
other scriptures ....
"scripture" is used 51 times describing the "old testament" (greek: graphe) so it is clear that it is a technical term.
It can only follow from that that Peter actually thought of Pauls letters as scripture too!
