Christians worship the one God who exists as three persons. The doctrine of the Trinity is, indeed, a mystery. However, the doctrine is essential. The Scriptures describe the one God who is three persons—Father, Son, and Spirit. These three persons are distinct from each other, but the three persons are also one God. The complexity of the Trinity rests on how one God can be three persons.
The Old Testament, though not as clearly as the New Testament, does reveal the Trinity. As Bavinck wrote, “The seeds that developed into the full flower of New Testament trinitarian revelation are already planted in the Old Testament. Elohim, the living God, creates by speaking his word and sending his spirit” (RD 2, 256).
God is clearly addressed as the Creator in Genesis 1. God is the deliverer in Exodus 3. The Trinity worked inseparably in all these things. It seems that it was the Father who said “let us create man in our own image” (Gen 1:26-27). The Father’s words to the Son are recorded in Psalm 110:1. The Spirit of God (the Holy Spirit) was at work in creation (Gen 1:2). When we read about “the angel of the Lord,” we see a person who is distinct from the Father but also is described as God (Gn 18; 22:11-18; 32:24-30; Ex 3:2-6; Is 63:9; and Zech 3:1-4).
How can there be three divine persons if there is only one God? There is one God. Christians do not worship three gods. Christians must only worship God (Matt 4:10; Dt 6:13), but our one God exists as three persons. The three persons of the Trinity share the same essence and will. There is no hierarchy. There is no “society” as we think of it. Instead, the three persons are one God. Hebrews 1 described it this way: He is the radiance of His glory and the exact representation of His nature (Heb 1:3). John said, “the Word was God” (Jn 1:1). On John 1:1, Michael Horton observed that, “The Word is simultaneously distinct from God the Father (“was with God”) and one in essence with the Father (“was God”).” Christians are commanded to baptize in the “name (singular) of the Father and of the Son and of the Holy Spirit” (Matt 28:20). The three persons have the same name meaning that they share the same divine essence.
In this way we begin to understand, though we will never comprehend, how we worship one God who exists (the technical word is “subsists”) as three persons. This doctrine is fundamental to Christianity. The Bible does not teach that the Father, Son, and Spirit are all one person. The Bible does not teach that Jesus was created or adopted into the divine family. The Father, Son, and Spirit are eternally one God.
The Biblical data was summarized as follows:
we worship one God in Trinity, and Trinity in Unity; Neither confounding the persons nor dividing the substance. For there is one person of the Father, another of the Son, and another of the Holy Spirit. But the Godhead of the Father, of the Son, and of the Holy Spirit is all one, the glory equal, the majesty coeternal. Such as the Father is, such is the Son, and such is the Holy Spirit. The Father uncreated, the Son uncreated, and the Holy Spirit uncreated. The Father incomprehensible, the Son incomprehensible, and the Holy Spirit incomprehensible. The Father eternal, the Son eternal, and the Holy Spirit eternal. And yet they are not three eternals but one eternal. As also there are not three uncreated nor three incomprehensible, but one uncreated and one incomprehensible. So likewise the Father is almighty, the Son almighty, and the Holy Spirit almighty. And yet they are not three almighties, but one almighty. So the Father is God, the Son is God, and the Holy Spirit is God; And yet they are not three Gods, but one God. So likewise the Father is Lord, the Son Lord, and the Holy Spirit Lord; And yet they are not three Lords but one LordAthanasian Creed
 Michael Horton, The Christian Faith: A Systematic Theology for Pilgrims on the Way (Grand Rapids, MI: Zondervan, 2011), 275.