Among other witnesses, one author has put together this list.... Can you add to it? See how many we can point to as attestations to the Gospels outside the New Testament itself.
I might point you to, among others, F. F. Bruce's
Jesus and Christian Origins Outside the New Testament...
- The Didache (written between AD 70 and 100 quotes the Gospels of Matthew and Luke. It also refers to the "gospels" (plural) revealing that there already more than one.
- Saint Clement (the fourth pope circa AD 96) in his epistle to the Corinthians contains ten quotations from both Matthew and Mark.
- The Epistle of St Barnabas (circa 90-130) quotes Matthew, Mark, and Luke.
- St Ignatius of Antioch (d. AD 108) quotes Matthew, Luke, and John.
- Papias (circa AD 120) spoke of all four Gospels and said that Matthew first wrote the words and acts of Christ in the Hebrew language which was later translated into Greek.
- St Justin Martyr knew all four Gospels and refers frequently to Luke.
- Tertullian (ca. AD 200) spoke of the Gospels "of Matthew and John the Apostles and Mark and Luke the disciples of Apostles."