Didache — Among the oldest Christian documents outside the New Testament — moral catechism, church manual, and the earliest surviving eucharistic prayers.
Marcion — Fragments and reconstructions from Marcion of Sinope, whose radical theology forced the proto-orthodox Church to define its own scriptural canon.
The Epistle of Barnabas — An anonymous early Christian epistle, traditionally but almost certainly wrongly attributed to Paul's companion Barnabas, arguing through dense allegorical reading of the Old Testament that Jewish ceremonial law was never meant to be kept literally.
The Epistle of Polycarp to the Philippians — Polycarp, bishop of Smyrna and a link to the apostle John himself, writes to the church at Philippi around the time of Ignatius's martyrdom, warning against greed, docetism, and the errors of a fallen presbyter named Valens.
The Epistle to Diognetus — An anonymous second-century defense of Christianity to an otherwise unknown inquirer named Diognetus — prized for its description of Christians as the soul of the world and its early theology of divine love.
The Epistles of Ignatius — The seven letters Ignatius of Antioch wrote under armed guard on his way to martyrdom in Rome, c. 107 CE, urging unity around bishop and eucharist against docetic teachers who denied Christ came in the flesh.
The First Epistle of Clement to the Corinthians — The oldest datable Christian writing outside the New Testament — the church at Rome's letter urging the church at Corinth to restore the presbyters its younger members had deposed, ending in the earliest surviving Christian liturgical prayer.
The Fragments of Papias — The surviving fragments of Papias of Hierapolis's lost five-book work on the sayings of Jesus, preserving early traditions about the origins of Matthew's and Mark's Gospels and a hearer's memory of the apostle John.
The Martyrdom of Ignatius — A companion-narrated account of Ignatius of Antioch's final journey to Rome and his death in the arena, traditionally read alongside his seven letters as the close of his story.
The Martyrdom of Polycarp — The oldest surviving eyewitness account of a Christian martyrdom — the church at Smyrna's letter describing the arrest, trial, and burning of its elderly bishop Polycarp around the middle of the second century.
The Second Epistle of Clement (So-Called) — The oldest complete Christian sermon to survive outside the New Testament — a mid-second-century homily on repentance and the resurrection of the flesh, wrongly attributed to Clement of Rome since ancient times.
The Shepherd of Hermas — One of the most popular texts of early Christianity, read as scripture by Irenaeus and Origen, presenting visions, commandments, and parables of a freed slave guided by an angel in the guise of a shepherd.