Trinity Sunday is the feast of the Holy Trinity — the one God in three Persons, Father, Son, and Holy Spirit. It falls on the first Sunday after Pentecost, completing the great arc of the liturgical calendar: the Father sends the Son (Christmas), the Son accomplishes redemption (Easter), the Spirit is given to the Church (Pentecost), and on Trinity Sunday the Church confesses the fullness of God as revealed through these mighty acts. Unlike most feasts, Trinity Sunday commemorates not an event but a doctrine — the central mystery of the Christian faith. The feast is comparatively late in origin. While Trinitarian theology was definitively settled at Nicaea (325) and Constantinople (381), a distinct feast of the Trinity did not become widespread until the medieval period. Individual bishops promoted it — Thomas Becket, consecrated on Trinity Sunday 1162, established it at Canterbury. Pope John XXII made it universal for the Western church in 1334. The BCP tradition, however, gave Trinity Sunday a structural importance beyond Rome's: in the English Prayer Book tradition, the Sundays of the long season after Pentecost are numbered 'after Trinity' rather than 'after Pentecost,' making Trinity Sunday the hinge of the entire second half of the church year. The BCP 2019 lists it among the seven Principal Feasts, and the Athanasian Creed — the most detailed creedal exposition of Trinitarian doctrine — is traditionally appointed for this day.

Trinity Sunday's liturgical tradition reflects the challenge of celebrating a doctrine rather than a narrative. The appointed readings across the three-year lectionary cycle present different facets of Trinitarian revelation: Nicodemus's nighttime conversation with Jesus (John 3), the baptismal commission 'in the name of the Father and of the Son and of the Holy Spirit' (Matthew 28), and Paul's Trinitarian benediction (2 Corinthians 13:14). The Athanasian Creed (Quicunque Vult), with its precise and paradoxical formulations — 'neither confounding the Persons nor dividing the Substance' — has been associated with this feast since at least the ninth century in the West, and the BCP 2019 includes it among its historical documents. The collect for Trinity Sunday is among the most theologically careful in the Prayer Book: 'by the confession of a true faith, to acknowledge the glory of the eternal Trinity, and in the power of your divine Majesty to worship the Unity.' Cranmer's 1549 language echoes here. The white or gold vestments used on this day reflect its status as a feast of the Godhead. In Anglican practice, numbering Sundays 'after Trinity' rather than 'after Pentecost' has given this feast a unique structural role — it names half the church year.

Justin was born around 100 AD in Flavia Neapolis (modern Nablus, Palestine) to a pagan family and received a thorough education in Greek philosophy. In his youth he studied the Stoics, Peripatetics, Pythagoreans, and Platonists, seeking truth through philosophical inquiry. He became a Platonist, hoping to achieve direct knowledge of God through philosophy. Then, according to his own account in the Dialogue with Trypho, he encountered an old man who redirected him toward the prophets of Israel and toward Christianity. Justin investigated Christian teaching and became convinced that 'this philosophy alone is safe and profitable.'

Justin moved to Rome and established a school where he taught Christianity as the true philosophy. His apologetic method was distinctive: rather than dismissing pagan philosophy, he argued that the divine logos (reason) had scattered seeds of truth throughout human philosophy and ethics before becoming incarnate in Christ. Thus Plato, the Stoics, and other philosophers had participated in the divine logos — but only in fragmentary fashion. In Christ, the full expression of divine wisdom had appeared.

This framework, known as the logos doctrine or logos spermatikos ('seeds of the Word'), became one of the most influential ideas in Christian theology. It provided a way to integrate Greek philosophy into Christian thought and to argue that Christianity fulfilled rather than contradicted human reason.

Justin's surviving works include his First Apology (addressed to the Emperor Antoninus Pius around 155 AD) and Second Apology (addressed to the Roman Senate), along with his Dialogue with Trypho (a Jewish interlocutor), which is the longest early Christian document and a foundational text for Christian-Jewish polemic.

The Apologies remain invaluable sources for understanding 2nd-century Christian worship: they describe the Sunday gathering, the reading of the 'memoirs of the apostles' (Gospels), the homily, the prayers of intercession, the offering of bread and wine, the eucharistic prayer, the distribution to the present and to the absent (deacons bringing the elements to those in prison or unable to attend). Justin's account of the Eucharist — 'Not as common bread nor common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh by the Word of God, had both flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word... is the flesh and blood of that Jesus who was made flesh' — articulates a high theology of the sacrament that would shape Christian eucharistic theology thereafter.

Justin was denounced as a Christian, tried before the Roman prefect, and beheaded around 165 AD. The Acts of Justin and his Companions, which records this trial and execution, survives as an authentic martyrdom account.

Traditionally, Justin is venerated as a saint and martyr who died defending the faith. Later hagiographic sources elaborate upon his life and teaching, but these expansions are minor compared to the full body of his own writings that have survived.