Irenaeus of Lyons, a disciple of Polycarp (himself a disciple of John), wrote his five-volume Against Heresies around 180 AD to refute Gnostic distortions of Christianity. He names the four canonical Gospels — Matthew, Mark, Luke, John — and argues theologically why there are exactly four. He preserves a chain of teaching from John to Polycarp to himself, and quotes extensively from earlier Christian writings.
Why this matters
The earliest writer to insist on the four canonical Gospels by name and to lay out the rule of faith (a creedal summary of Christian teaching). His chain — John → Polycarp → Irenaeus — connects orthodox teaching directly back to an apostle.
Scripture references
MatthewMarkLukeJohn1 JohnJude
Location
Various