The three Persons in one God. God is one in nature but in that one God are three distinct persons, the Father, the Son who proceeds from the Father by generation, and the Holy Spirit who proceeds from the Father and the Son, as from one principle, by spiration.
Implied in OT.
Gen. 1:26 – Let us make man in our image.
Gen. 11:5-7 – let us go down and confound their tongue.
Ps. 2:7 – You are my son.
St. Cyprian of Carthage [AD 251/156] “The Lord says, “I and the Father are one; ” and again it is written of the Father, and of the Son, and of the Holy Spirit, “And these three are one”. And does any one believe that this unity which thus comes from the divine strength and coheres in celestial sacraments, can be divided in the Church, and can be separated by the parting asunder of opposing wills? He who does not hold this unity does not hold God’s law, does not hold the faith of the Father and the Son, does not hold life and salvation”.