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\section{Theology in the patristic era}
Christianity is culturally rooted in both Judaism and Greco-Romanism: since
Jesus was a Jew, many patterns from the Hebrew Bible are taken over by the
evangelists. Also, early Christian theologians were in constant dialogue with
the Greek intellectual tradition, that is available around the Mediterranean
Sea thanks to Hellenisation.

The \index{Apostolic fathers}apostolic fathers are those theologians that were
active in the late first to mid-second century. They are mainly concerned with
church organisation. \index{Clement}Clement advocates a strong hierarchy.
\index{Ignatius}Ignatius coins the term \index{Catholicity}\emph{catholicity}
to stress the unity and universality of the church.

Another topic is \index{Christology}christology. Both Clement and Ignatius
assert that Jesus existed before Creation, in contrast to the Ebionites, who
claimed Jesus was but a Jewish teacher. On the other hand, Ignatius also
affirms Jesus' true humanity, opposing \index{Docetism}docetism, the view that
Jesus only seemed human.

In dialogue with the Roman empire, the idea of \index{Martyrdom}martyrdom and
the apologetics\index{Apologetics} came up. The main charges levelled at early
Christians were: atheism (because they refused to worship the Roman gods);
cannibalism (because of the Eucharist); and sexual immorality (because of the
term `brothers and sisters' for everyone). Apologists did not only counter
these charges but also tried to argue for the reasonability of the new faith.
\index{Justin Martyr}Justin Martyr is an example of this: he developed the
\index{Logos!Doctrine}Logos doctrine, which claims that the logos (as the
rational structure of the cosmos) became incarnate in Christ, implying that
Christianity is the highest form of truth.

In this first time, \index{Orthodoxy}orthodoxy came up as opposing
\index{Heresy}heresies. Some heresies: \index{Ebionism}Ebionism tried to
preserve the Jewish purity of the church, emphasising monotheism and reaching
out to adoptionism for christology. \index{Docetism}Docetism taught that
Christianity should separate entirely from its Judaic roots, rejecting the
Hebrew Bible. Yahweh is an inferior God, reigning through justice and law in
the evil realm of matter, while Jesus is a true God, working through grace and
love in the good spiritual realm. \index{Modalism}Modalism is the view that God
is a single monad expressing itself in three operations.

Most importantly, \index{Gnosticism}gnosticism held that salvation comes via a
secret given through special revelation. The world is created by an evil
\index{Demiurge}\emph{Demiurge}. Spiritual souls are trapped in a body but can
escape by the grace of Jesus Christ. Christ then has to be fully divine (He
cannot be evil), and therefore gnosticism is partially docetic.

\index{Irenaeus}Irenaeus counters gnosticism by stressing \index{Creation!Ex
nihilo}creatio ex nihilo and insisting that God keeps interacting with the
world. Humanity goes through a process of \emph{maturation} that would lead to
the choice to remain faithful. This process has been stopped by Adam and Eve's
\index{Sin}sin, which is why redemption or \emph{recapitulation} is needed. In
the end, salvation is the fulfilment of creation, not an escape from it.

\index{Tertullian}Tertullian attempts to free Christianity from pagan
philosophy (`What has Athens to do with Jerusalem?'). He also coins the term
\index{Trinity}\emph{trinitas}, which he sees as three persons in one
substance.

\index{Origen}Origen is ambiguous: sometimes he seems to claim that the Son is
eternal and has his personal being from the Father (eternal begottenness); at
other times he suggests a subordinationist arrangement among the three members
of the Trinity.

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In the fourth century, Christianity became the state religion under
Constantine. After this era, theology becomes preoccupied with the doctrines of
the Trinity and of Christ.

Also, the NT canon was closed, using four criteria:
\index{Apostolicity}apostolicity (should reflect apostolic teaching);
\index{Catholicity}catholicity (should be relevant to the whole church);
\index{Orthodoxy}orthodoxy (should agree to the rules of faith, the creeds);
and \index{Tradition}traditional usage (should be used widely in the life of
the church). Much of the canon was already decided on in the second century,
but the first list of the NT canon as we know it today dates from 367.

On the \index{Trinity}Trinity, \index{Arianism}Arius proclaimed that the Son
was a creature. Alexander and \index{Athanasius}Athanasius argued that if Jesus
wasn't truly God, he couldn't accomplish redemption. In \index{Nicea}Nicea in
325 the debate was settled: the Son is of the same essence
(\index{Homoousios}\emph{homoousios}) as the Father. Only the Creator can
redeem creation. Some eastern church leaders then proposed
\index{Homoiousios}\emph{homoiousios}, `of similar essence', but this was
condemned in \index{Constantinople}Constantinople in 381. There are three
divine persons (hypostaseis or personae) who share one essence or substance
(ousia or substantia).

On Christ, the question was how He can be both divine and human. The
\index{Alexandria!School of}Alexandrian school (notably
\index{Apollinarius}Apollinarius) emphasised his divinity and the unity of his
person: the Logos overshadowed Jesus' humanity; the \index{Antioch!School
of}Antiochene school (Theodore of Mopsuestia) his humanity and his two natures:
how could a non-human ever redeem humanity? The Alexiandrans proposed the term
\index{Theotokos}\emph{Theotokos}, `God-bearer', intensifying the debate. This
idea was confirmed at \index{Chalcedon}Chalcedon in 451, where it was also said
that Christ is \emph{one} person with \emph{two} natures.

\medskip
The \index{Cappadocians}Cappadocians extended the logic of Nicea (if Jesus
brings salvation, and salvation is deification, then Jesus must be divine) to
the Spirit (if the Spirit brings sanctification, \dots), thus affirming the
oneness of the whole Trinity. They also distinguished
\index{Hypostasis}hypostasis and \index{Ousia}ousia: the latter is the essence
common to the three members of the Trinity, while the first is their
individuated existence. Gregory of Nyssa compares the Trinity to Peter, James
and John, sharing humanity.

\index{Augustine}Augustine proposes a psychological analogy for the Trinity
rather than a social one. The Trinity is then compared with mind, knowledge and
love.

The \index{Donatism}Donatists claimed that bishops who seized to persecutions
were traitors. Augustine counters by saying that even though the Church is
built up from sinful humans, it is still holy because it participates in
Christ's holiness.

The \index{Pelagianism}Pelagians said that humans can, also after Adam's sin,
achieve true perfection because of free will. Augustine counters by claiming
that human nature is infected by \index{Sin!Original}original sin.
