The Westminster Shorter Catechism

Q6. How many persons are there in the Godhead?

A. 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.[1]

Proofs

[1] Deuteronomy 6:4; Jeremiah 10:10; Matthew 28:19; 1 John 5:7.

Comments

The Scripture not only teaches us that there is only one God (e.g., Deut 6:4), it also teaches us that there are three Persons in the Godhead. This is so because the Father is God (Isa 63:16); the Son is God (Jn 1:1–2; Isa 9:6; Rom 9:5; 1 Tim 3:16 Phil 2:6; 1 Jn 5:20; etc.); the Holy Spirit is a person (Jn 14:26; Acts 16:7; 5:3, 9; Eph 4:30; etc.) and He is God (Ps 139:7–9; Heb 9:14; etc.). Moreover, the Scripture clearly teaches us that the Father, the Son and the Holy Spirit are distinct persons. This is hinted in the OT with the use of the plural pronoun (e.g., Gen 1:26–27), but is most clearly seen in the baptism of Jesus (Mt 3:16–17), where we see Jesus being baptised, the Holy Spirit descending and the Father speaking from heaven at about the same time. Thus the Father is not the Son nor the Holy Spirit, and the Son is not the Holy Spirit. This means that the popular illustrations used to teach the doctrine of the Trinity, such as the three states of water or the father-son-uncle roles are simply heretical (an ancient heresy known as Sabellian modalism). There are simply illustrations not suitable to explain the Trinity.

While we are unable to fully comprehend the Trinity, we may apprehend sufficiently to know that God is one in being or substance and three in person or subsistence. And since each person in the Godhead is equally God—equal in power and glory,—each must be worshipped and honoured alike.