umanity corresponds to
Irenaeus' doctrine of the God-man. The historical importance of this
author lies in the development of the Christology. At the present day,
ecclesiastical Christianity, so far as it seriously believes in the
unity of the divine and human in Jesus Christ and deduces the divine
manhood from the work of Christ as his deification, still occupies the
same standpoint as Irenaeus did. Tertullian by no means matched him here;
he too has the formula in a few passages, but he cannot, like Irenaeus,
account for its content. On the other hand we owe to him the idea of the
"two natures," which remain in their integrity--that formula which owes
its adoption to the influence of Leo I. and at bottom contradicts
Irenaeus' thought "the Son of God became the Son of man," ("filius dei
factus filius hominis"). Finally, the manner in which Irenaeus tried to
interpret the historical utterances about Jesus Christ from the
standpoint of the Divine manhood idea, and to give them a significance
in regard to salvation is also an epoch-making fact.

"Filius dei filius hominis factus," "it is one and the same Jesus
Christ, not a Jesus and a Christ, nor a mere temporary union of an aeon
and a man, but one and the same person, who created the world, was born,
suffered, and ascended"--this along with the dogma of God the Creator is
the cardinal doctrine of Irenaeus:[577] "Jesus Christ truly man and truly
God" ("Jesus Christus, vere homo, vere deus").[578] It is only the
Church that adheres to this doctrine, for "none of the heretics hold the
opinion that the Word of God became flesh" ("secundum nullam sententiam
haereticorum verbum dei caro factum est").[579] What therefore has to be
shown is (1) that Jesus Christ is really the Word of God, i.e., is God,
(2) that this Word really became man and (3) that the incarnate Word is
an inseparable unity. Irenaeus maintains the first statement as well
against the "Ebionites" as against the Valentinians who thought that
Christ's advent was the descent of one of the many aeons. In opposition
to the Ebionites he emphasises the distinction between natural and
adopted Sonship, appeals to the Old Testament testimony in favour of the
divinity of Christ,[580] and moreover argues that we would still be in
the bondage of the old disobedience, if Jesus Christ had only been a
man.[581] In this connection he also discussed the birth from the
virgin.[582] He not only proved it from prophecy, but his rec