The testimony of eyewitnesses, not necessarily apostles, was valued in compiling the Christian record. It becomes tricky, though, because New Testament materials weren't definitively selected until the 4th century. That's a long way from the generation of eyewitnesses and required sifting through hearsay, because many texts were not initially attributed to anyone. Authorship, therefore, was not as important as authority. Who wrote the testimony wasn't as critical as who was discerned as the original source.
Take Mark. His name didn't appear in the gospel later attributed to him. And who is "Mark" anyway? Not one of the 12 apostles. Paul had a companion John Mark mentioned in the Acts of the Apostles and several letters. Another Mark is described as close to Peter in the First Letter of Peter. It's possible this is the same person, in which case his testimony derives from an eyewitness and another early source.
Matthew's gospel followed, employing Mark heavily (55 percent of Matthew comes from Mark) and sharing another unknown source (25 percent) with Luke. But who combined these sources with additional material for the final version is uncertain. The apostle Matthew, a.k.a Levi the tax collector, is unlikely. Why would an apostle copy from a non-eyewitness rather than write from memory? Also, this gospel is quite rabbinical in style. A former rabbi or scribe was most likely the final compiler. He may have taken the unique material (20 percent) from sayings attributed to the apostle Matthew.
Luke borrowed from Mark (over 40 percent) and shared material with Matthew. But 35 percent of Luke came from somewhere else. The biblical Luke was a Syrian physician converted by Paul, so at best his story comes to us thirdhand. Even Paul was not a first-generation apostle. Scholars are divided as to whether this Luke was responsible for both Luke and Acts; the arguments are intriguing either way.
Few would attribute the last gospel directly to John, son of Zebedee. It was written very late in the 1st century, when the apostles might be presumed dead. Its authorship could be traced to a community taught by the apostle, or by another well-known Christian teacher named John in Asia Minor at the same time. Many scholars attribute the gospel and letters of John to a group rather than an individual. Ninety percent of this material is unique to the fourth gospel.
• Mark 1:1; Luke 1:1-4; John 21:20-25; Acts 1:1-2; Acts 12:12; Colossians 4:10; 2 Timothy 4:11; 1 Peter 5:13
• “Who really wrote the gospels, and why should we care?” by Felix Just, S.J.
• Four Portraits of Jesus: Studies in the Gospels and Their Old Testament Background by Elizabeth E. Platt (Paulist Press, 2004)
• The What Are They Saying About . . . series: WATSA Mark, Matthew, Luke, and John by various authors (Paulist Press)