How many persons are there in the Godhead?
There be three persons in the Godhead, the Father, the Son, and the Holy Ghost; and these three are one true, eternal God, the same in substance, equal in power and glory; although distinguished by their personal properties.1
Scripture proofs
The Westminster Assembly's proof texts (KJV). The bracketed numbers in the answer above mark the clause each set of references supports; expand a row to read the verses.
1 1 John 5:7; Matt. 3:16–17; Matt. 28:19; 2 Cor. 13:14; John 10:30
For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.
Matt. 3:16 And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him:
Matt. 3:17 And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased.
Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:
The grace of the Lord Jesus Christ, and the love of God, and the communion of the Holy Ghost, be with you all. Amen.
I and my Father are one.
This doctrine across the Standards
Where the Confession and the Westminster Shorter Catechism treat the same matter, so the question can be studied alongside its parallel statements.
In the Westminster Confession
II.3 In the unity of the Godhead there be three persons, of one substance, power, and eternity: God the …
In the unity of the Godhead there be three persons, of one substance, power, and eternity: God the Father, God the Son, and God the Holy Ghost. The Father is of none, neither begotten nor proceeding; the Son is eternally begotten of the Father; the Holy Ghost eternally proceeding from the Father and the Son.
→ II.3 · Of God, and of the Holy TrinityVIII.2 The Son of God, the second person in the Trinity, being very and eternal God, of one substance, …
The Son of God, the second person in the Trinity, being very and eternal God, of one substance, and equal with the Father, did, when the fulness of time was come, take upon him man's nature, with all the essential properties and common infirmities thereof, yet without sin: being conceived by the power of the Holy Ghost in the womb of the Virgin Mary, of her substance. So that two whole, perfect, and distinct natures, the Godhead and the manhood, were inseparably joined together in one person, without conversion, composition, or confusion. Which person is very God and very man, yet one Christ, the only mediator between God and man.
→ VIII.2 · Of Christ the MediatorIn the Westminster Shorter Catechism
Q.5 Are there more Gods than one?
There is but one only, the living and true God.
→ Open Q.5Q.6 How many persons are there in the Godhead?
There are three persons in the Godhead; the Father, the Son, and the Holy Ghost; and these three are one God, the same in substance, equal in power and glory.
→ Open Q.6