What is Christianity? The question is harder than it looks. A religion of two billion adherents has no single center: it is a collection of texts, a set of doctrines, a family of institutions, and a two-thousand-year history of interpretation — and these four layers do not always agree. The New Testament knows nothing of the Trinity as a formulated doctrine; the doctrine knows nothing of the historical contingencies that produced it; the churches that guard the doctrine have condemned each other over its wording. Any definition of Christianity is therefore already a position within Christianity.
This section is organized around that difficulty. Rather than beginning with a definition, it presents the layers separately and lets the reader see how they fit together — and where they don't.
The texts. Christianity is, first, a religion of a book, though which book was itself contested for centuries. The canonical writings — the Gospels, the letters of Paul — stand alongside texts the early church excluded: the Gospel of Thomas, the Nag Hammadi writings. The exclusions are as instructive as the inclusions. A canon is not a library; it is a decision about authority, and the excluded texts show what the decision was directed against.
The message. At the core stands a teaching that can be stated briefly and has resisted every brief statement: that God has acted in a particular human life, and that this act changes the relation between law, sin, and grace. Paul's Letter to the Romans works out the logic of this claim with a rigor that still deserves philosophical attention — his analysis of the divided will in chapter 7 anticipates problems that psychoanalysis would rediscover, in different vocabulary, nineteen centuries later.
The history. Doctrine did not fall from heaven; it was hammered out in councils, schisms, and reformations. The section follows this history through its figures: Aquinas, who gave the Incarnation its most systematic philosophical form; Francis of Assisi, who answered the institutional church with a practice rather than a theory; Luther, whose redefinition of faith split Western Christendom and, arguably, inaugurated the modern subject.
The philosophical reception. Finally, Christianity has always provoked argument from outside and within: proofs and disproofs of God's existence, C.S. Lewis's moral argument, the contemporary encounter with "New Age" spirituality. These debates belong to the subject, because a religion that makes truth-claims invites — and must survive — examination.
Since no summary can speak for all Christians, the following list states the maximal case — the requirements of the Catholic Church, the largest and doctrinally most articulated body. Protestant, Orthodox, and other Christian communities would revise this list at various points; the revisions are themselves part of the story told in this section.
That there is one supreme, eternal, infinite God, the Creator of heaven and earth.
That the good will be rewarded by him forever in heaven, and that the wicked who die unrepentant will be punished forever in hell.
That in the Holy Trinity there are three Persons, coeternal, coequal: God the Father, God the Son, and God the Holy Spirit.
That the Second Person of the Holy Trinity became man and died on the cross to save us.
The tenets of the Apostles' Creed.
The Commandments of God and of the Church.
That the seven sacraments were instituted by Christ to give us grace; especially, that Baptism is necessary and that the Eucharist is a pledge of our future glory.
That Sacred Tradition and Sacred Scripture, which together form one sacred manifestation of the Word of God, are entrusted to the Church.
That God teaches us also through the Church, who in her teaching cannot deceive us or be deceived.
Notice the structure of the list: it moves from metaphysics (God, Trinity, Incarnation) through ethics (commandments) to institution (sacraments, Church, tradition). The order is not accidental — it is the architecture of Catholic self-understanding, in which the Church is not an afterthought to the message but its authorized bearer. Luther's protest, documented elsewhere in this section, was directed precisely at this final step.
Canonical Texts — The New Testament · Sermon on the Mount · The Lord's Prayer · Magnificat · Paul: Romans on Law and Sin
Non-Canonical Texts — The Gospel of Thomas · The Thunder, Perfect Mind ("I was sent forth from the power")
The Message and Its Theology — The Message of Jesus · Aquinas: The Incarnation
History and Figures — History of Christianity · The first 500 Years · Martin Luther · St. Francis and the Franciscans
Christianity and Philosophy — Debate on the Existence of God · C.S. Lewis: The Law of Human Nature · Christian Reflections on "New Age"
Related elsewhere on this site: Meister Eckhart (under Mysticism) stands in the German lineage that leads to Luther. Ecclesiastes belongs to the Hebrew Bible and is treated under Judaism. General arguments for the existence of God are examined under Theology.