God & Reason
The Holy Trinity
God is one in nature and three in Persons—Father, Son, and Holy Spirit—a central mystery revealed by Christ himself and confessed by the Church from the beginning.
Catholic teaching
The Church teaches that there is only one God, yet in God there are three distinct, co-equal, and co-eternal Persons: the Father, the Son, and the Holy Spirit. Each Person is fully God, not a part or a mode of God, and each is really distinct from the others by their relations of origin: the Father begets the Son, the Son is eternally begotten, and the Holy Spirit proceeds from the Father and the Son. This is not tritheism (three gods) nor modalism (one God wearing three masks), but a single divine nature subsisting in three Persons.
Biblical foundation
At Jesus's baptism, the Father speaks from heaven, the Son stands in the Jordan, and the Spirit descends as a dove—three distinct Persons acting together. Jesus commands baptism 'in the name' (singular) of the Father, and of the Son, and of the Holy Spirit, implying one shared divine name and nature. Paul closes 2 Corinthians with a threefold blessing invoking all three Persons as a single source of grace.
Scripture references
Historical evidence
Trinitarian language appears from the earliest post-apostolic writings. The Didache, dated to the late first or early second century, already prescribes baptism 'in the name of the Father, and of the Son, and of the Holy Spirit.' Around AD 180, St. Theophilus of Antioch is the first to use the Greek word 'trias' (Trinity) explicitly of God. The doctrine was formally articulated against the Arian denial of Christ's full divinity at the Council of Nicaea (325) and against denials of the Spirit's divinity at the Council of Constantinople (381), producing the Nicene-Constantinopolitan Creed still used in the liturgy today.
The Church Fathers
“The three days before the luminaries were created are types of the Trinity, of God, and His Word, and His Wisdom.”
“The unity, unfolding itself in a trinity... three, however, not in condition, but in degree; not in substance, but in form; not in power, but in aspect.”
“There is, then, a Triad, holy and complete, confessed to be God in Father, Son, and Holy Spirit, having nothing foreign or external mixed with it.”