God is one in essence, yet distinguished in three persons

The Belgic Confession of Faith

Article 8: God is One in Essence, Yet Distinguished in Three Persons

According to this truth and this Word of God, we believe in one only God, who is the one single essence, in which are three Persons, really, truly, and eternally distinct according to their incommunicable properties; namely, the Father, and the Son, and the Holy Spirit. The Father is the cause, origin, and beginning of all things visible and invisible; the Son is the word, wisdom, and the image of the Father; the Holy Spirit is the eternal power and might, proceeding from the Father and the Son. Nevertheless, God is not by this distinction divided into three, since the Holy Scriptures teach us that the Father, and the Son, and the Holy Spirit have each His personality, distinguished by Their properties; but in such wise that these three Persons are but one only God.

Hence, then, it is evident that the Father is not the Son, nor the Son the Father, and likewise the Holy Spirit is neither the Father nor the Son. Nevertheless, these Persons thus distinguished are not divided, nor intermixed; for the Father has not assumed the flesh, nor has the Holy Spirit, but the Son only. The Father has never been without His Son, or without His Holy Spirit. For They are all three co-eternal and coessential. There is neither first nor last; for They are all three one, in truth, in power, in goodness, and in mercy.