The earliest heresies — the Ebionites and the Docetists — and the question beneath all the others: how did the church know which Jesus was the real one? Trace the line back to the apostles.
In Lesson 3 the fight was "how is the Son related to the Father?" (Modalism vs. Arianism). But that's already the second question. The first one — the one the apostles' own students faced — was more basic still: was Jesus truly God, and truly man? Here too there were two ditches, pulling in opposite directions.
Jesus was a righteous human — the natural son of Joseph and Mary — who was adopted as Messiah because he kept the Law perfectly. No virgin birth, no pre-existence, not God.
"A great prophet, nothing more."
✗ Loses the deity of Christ
The eternal Son of God genuinely became flesh — really born, really hungry, really crucified — without ceasing to be fully God. Both held at once.
"The Word became flesh." (John 1:14)
✓ Holds both deity AND humanity
Christ is so purely divine he could never touch real flesh; his body was a phantom. He only appeared to be born, to eat, to suffer, to die.
"It only looked like a body."
✗ Loses the humanity of Christ
Ebionism keeps the man and drops the God · Docetism keeps the God and drops the man · Orthodoxy keeps both. Docetism · DOH-suh-tiz-um
The Ebionites EE-bee-uh-nites didn't drop from the sky either. Their root is the very dispute you already meet in the New Testament: the Judaizers JOO-day-eye-zers — Jewish believers who insisted Gentile Christians must be circumcised and keep the whole Law of Moses (the fight of Acts 15 and Paul's letter to the Galatians). After the Jerusalem Temple fell in AD 70, a strand of strict Jewish-Christians hardened into a distinct sect.
Their package was consistent: Jesus is the natural son of Joseph (so no virgin birth), a man uniquely filled by God and adopted at his baptism — a view called adoptionism. They kept circumcision and the Mosaic Law, treated Paul as an apostate who had abandoned the Law, and used a single, edited Hebrew gospel close to Matthew. (Britannica; Catholic Answers.)
If Ebionism was the over-correction of the Jewish world (guard the one God; Jesus is just his prophet), Docetism was the over-correction of the Greek world. Greek philosophy widely assumed that matter is low and spirit is pure — so a truly divine being could never soil himself with real flesh. The name comes from Greek dokeō do-KEH-oh, "to seem." Christ only seemed to have a body.
This one was answered fast — by the apostles' own circle. Ignatius of Antioch (a disciple of John, whom you met in Lesson 3) insisted Christ was "truly born… truly persecuted… truly crucified," not "in mere appearance." And the apostle John had already drawn the line:
Every spirit that confesses that Jesus Christ has come in the flesh is from God, and every spirit that does not confess Jesus is not from God. — 1 John 4:2–3; see also Ignatius, To the Smyrnaeans
Here is the question you really asked. With Ebionites pulling one way and Docetists/Gnostics the other — each quoting Scripture, each claiming to be the true church — how did the orthodox side know it was right, and prove it? Their answer was not "we argue best" and not "we have a secret." It was: our teaching is the one you can trace, in the open, all the way back to the apostles.
Around AD 180, Irenaeus ih-reh-NAY-us of Lyon laid this out in his great book Against Heresies (Adversus Haereses · ad-VER-sus hye-REH-sees). He gave a three-legged test:
The writings the apostles actually left — the four public Gospels and the letters — not a secret text known to a few.
The short, shared summary of belief confessed at baptism in every church alike — the seed of what becomes the creeds.
You can name the unbroken line of bishops back to an apostle in the great public churches. The teaching has a paper trail.
His knockout blow against the Gnostics: they claimed a secret tradition Jesus told only to the inner few. Irenaeus answered — if the apostles had hidden truths, they would above all have handed them to the very men they appointed to lead the churches after them. Yet those public successors teach nothing of the sort. Heresy is always new and partial (it drops half of Jesus); the apostolic faith is old and whole.
And Irenaeus was himself a piece of living evidence — only two handshakes from Jesus:
Irenaeus tells us he listened to Polycarp as a boy, and Polycarp had "conversed with John and with the others who had seen the Lord." That is the chain in the flesh. (Irenaeus, Against Heresies III.3; Apostolic Succession sources.)
Classify each statement. Instant feedback below each one.
Read Irenaeus in his own words — short and striking: Against Heresies, Book III, ch. 3, where he argues the apostolic succession and names the line of bishops at Rome. This is the original of everything in Thread 3.
Want the difference between Ebionite adoptionism and Arianism (both "low," but not the same)? Curious how a modern group can be functionally Ebionite or Docetic today? Or how Protestants square sola scriptura with Irenaeus's appeal to succession?
Message me — I'm your teacher. When you're ready, say next. A natural follow-on is
"How we got the New Testament" — the canon — since Marcion and the Gnostics forced the church
to say which books were apostolic.