Monotheistic religions hold in common the belief that there is only one God and therefore only one divine essence. The identity of this God differs from religion to religion, but fundamental agreement exists that there is only one God, no matter who He happens to be.
Christian monotheism, however, does differ from other monotheistic faiths in its understanding of how this essence is possessed by God, even though Christianity agrees that there is only one God and only one divine essence. Other monotheistic faiths teach that the one divine essence belongs to only one divine person. Historic, biblical, orthodox Christian monotheism, however, says that the one divine essence belongs equally to three distinct divine persons. God is not merely Allah, for example; He is Father, Son, and Holy Spirit.
In other words, Father, Son, and Spirit are all fully divine, possessing identical divine attributes. Being fully equal, none is more or less God than the other. And this is our confession based on much biblical evidence.
First, some theologians have noted the Old Testament use of Elohim, one of the Hebrew terms translated into English as “God.” Though it is often used as a singular in Hebrew grammar, Elohim is actually plural in form. So, some have seen in the word itself a hint of plurality in God. It should be noted that the mere word itself does not necessitate a reference to divine plurality, and it would be insufficient by itself to give us a God who is both one and many. Its use, however, is congruent with the doctrine of the Trinity.
Psalm 110, which makes a distinction between the “LORD” (Yahweh) and the “Lord” (Adonai), also points us in the direction of Trinitarianism. There is an allusion to two divine persons, and Jesus brings this out in His use of the text to show His opponents that He is far more than a mere human son of David (Matt. 22:41–46).
Although there are various hints at God’s Trinitarian existence in the Old Testament, we do not get a full-orbed revelation of the Trinity until the New Testament. John 1:1–18, for example, is a clear teaching on the deity of Christ, the Word who in the beginning was God and was with God. There are also texts wherein the three persons are so closely associated so as to imply that all three are fully divine (for example, 2 Cor. 13:14). Clearly, the Bible assumes and teaches that God is triune.