Serious theology, at the depth you choose.

Christians believe in one God who is Father, Son and Holy Spirit. Not three gods, and not one God playing three roles — three distinct persons sharing one divine life. It's the Christian way of saying God is love even before creation existed: love between the Father, Son and Spirit.

The doctrine guards against two errors: tritheism (three gods) and modalism (one person in three modes). It was formalised at Nicaea (325) and Constantinople (381), using ousia (essence) for what is one and hypostasis (person) for what is three. Each person is fully God; they are distinguished by their relations — the Father begets, the Son is begotten, the Spirit proceeds.

Key debates include the filioque (whether the Spirit proceeds from the Father 'and the Son', added to the Western Creed and a factor in the East–West schism); the Cappadocian Fathers' development of hypostasis; Augustine's psychological analogy in De Trinitate; perichoresis (mutual indwelling); and modern 'social Trinitarianism' (Moltmann, Zizioulas) versus classical accounts emphasising divine simplicity. Karl Rahner's axiom that the 'economic' Trinity is the 'immanent' Trinity reshaped twentieth-century discussion.

Christians believe Jesus is God the Son who became a real human being — born, hungry, tired, crucified — without stopping being God. That is what 'incarnation' means: God 'in the flesh'.

The Council of Chalcedon (451) defined Christ as one person in two natures, divine and human, 'without confusion, change, division or separation'. Rejected views include Arianism (the Son is created), Apollinarianism (no human mind), Nestorianism (two persons) and Eutychianism (natures merged). The Oriental Orthodox churches did not accept Chalcedon's wording, preferring 'one incarnate nature of God the Word' (miaphysitism).

Advanced questions include the communicatio idiomatum (how attributes of each nature are predicated of the one person); dyothelitism (two wills, affirmed at Constantinople III, 681); kenotic Christology based on Philippians 2:7; and the relationship between the 'historical Jesus' of critical scholarship and the Christ of faith. Recent bilateral statements between Eastern and Oriental Orthodox suggest the Chalcedonian divide is largely terminological.

Salvation means God rescuing people from sin and its consequences and bringing them back into friendship with him through Jesus. Every major Christian tradition agrees it is a gift of grace, not something we earn.

Traditions differ on how it is received: Protestants emphasise justification by grace through faith alone (sola fide); Catholic teaching holds justification includes inner renewal and cooperation with grace through the sacraments; Orthodoxy frames salvation as theosis — progressive union with God. The 1999 Joint Declaration on the Doctrine of Justification (Catholic–Lutheran) found a shared basic understanding.

Deeper questions: Augustine versus Pelagius on grace and free will; Calvinist and Arminian disagreement (the Synod of Dort, 1618–19) on election, the extent of the atonement and perseverance; theories of atonement (Anselm's satisfaction, penal substitution, Christus Victor, moral influence); the New Perspective on Paul (Sanders, Dunn, Wright) and its reading of 'works of the law'.

The Holy Spirit is God with us and in us — the third person of the Trinity. The Spirit draws people to faith, helps them pray, gives them gifts to serve others, and grows qualities like love, joy and peace in them.

Pneumatology covers the Spirit's person (fully divine, affirmed at Constantinople 381) and work: regeneration, indwelling, sanctification, gifts (charismata) and fruit. Traditions connect the Spirit with baptism and confirmation/chrismation; Pentecostal and charismatic movements emphasise post-conversion empowerment and gifts such as tongues and healing.

Advanced topics: the filioque controversy and Photius; Basil the Great's On the Holy Spirit; the Spirit and the church in Orthodox ecclesiology; cessationism (B. B. Warfield) versus continuationism; the global growth of Pentecostalism since Azusa Street (1906) and its theological implications.

The Church is not a building but the community of people who follow Jesus. Christians gather to worship, learn, pray, share bread and wine, and care for one another. Baptism and Communion are practices nearly all Christians share.

Ecclesiology asks what the Church is and how it is structured. Catholic teaching locates the Church's unity in communion with the Pope and bishops; Orthodoxy in the communion of local churches with their bishops; Protestants in the gospel rightly preached and sacraments rightly administered. Catholics and Orthodox recognise seven sacraments/mysteries; most Protestants two.

Debated questions: apostolic succession and validity of orders; the four 'marks' (one, holy, catholic, apostolic) in the Creed; sacramental theology (ex opere operato, memorialism, real presence); the ecumenical movement and documents like Baptism, Eucharist and Ministry (WCC, 1982).

Christians believe the Bible is inspired by God and tells the true story of God's dealings with the world, centred on Jesus. It is a library of 66 books (more in Catholic and Orthodox Bibles) written over many centuries.

Traditions agree Scripture is authoritative but differ on its relation to Tradition and the Church: sola scriptura (Protestant), Scripture and Tradition interpreted by the Magisterium (Catholic), Scripture within Holy Tradition (Orthodox). They also differ on the Old Testament canon (see the Bible hub).

Advanced study covers doctrines of inspiration, inerrancy and infallibility (e.g. the Chicago Statement, 1978, versus Dei Verbum, 1965); textual criticism and manuscript traditions; historical-critical, canonical and theological interpretation; and the formation of the canon from the early church to Trent (1546).

Christians believe Jesus will return, the dead will be raised, evil will be judged, and God will make all things new. Their hope is not just 'going to heaven' but a renewed creation where God lives with his people.

Major views on the millennium of Revelation 20: amillennial (Catholic, Orthodox, many Reformed and Lutheran), postmillennial, and premillennial (including dispensational premillennialism, popular among many American evangelicals). Traditions also differ on purgatory and prayer for the dead.

Advanced discussion includes inaugurated eschatology ('already and not yet'); preterist and futurist readings of Revelation; debates over hell (eternal conscious punishment, conditional immortality, Christian universalism); and the 'intermediate state' between death and resurrection.