The Holy Trinity
Nowhere stated as such in scripture, the doctrine was fixed in the fourth century: Nicaea (325) declared the Son of one being with the Father, against Arius's view that he was a created being, and Constantinople (381) affirmed the Spirit's full divinity, completing the formula of one God in three persons.
The Creed of Nicaea (325) and the Niceno-Constantinopolitan Creed (381); J. N. D. Kelly, Early Christian Doctrines (1958), chs. 9–10
The doctrine that God is one being in three persons — Father, Son, Holy Spirit — inseparable and co-equal.