What is the 'trinity'?
Does the Bible speak of God in plural?
Biblically, Christians believe in one God (monotheism--'one God') eternally existing in three Persons. Christians do not believe in three gods (polytheism--'many gods') or one God acting as three different Persons (modalism). Though the word 'trinity' is not in the Bible, the concept is used to attempt to understand the biblical teaching of what and who makes up 'God'. Throughout the Old Testament (Jewish Torah/Tanakh) the Hebrew word for 'God', elohim, is a plural, masculine noun used over 2500 times, which, if we didn't know that God was 'one' could be translated 'gods'. Elohim is primarily used in reference to the true God and often in conjunction with a singular verb bara ('he created'--Gen. 1:1). This stands in stark contrast to usage of the singular noun for God (el, eloah) which is comparatively rarely used.
While some have claimed that 'elohim' is a word referring to God's overwhelming majesty, it doesn't make sense with nearly all contexts where the word describing God is used. Interestingly, the plural form of God, elohim, is used when God creates man in His 'image' (singular) yet refers to Himself with a plural pronoun (Us, Our, We--Gen. 1:26; 3:26; 11:7; Is. 6:8). As humans are made in God's image and angels are never consulted by God for anything, God was talking to/among Himself (Gen. 1:27). So the plural God creates man in His singular image! This means God's personhood is multiple, but their essence is singular.
The Jewish Shema (Hebrew word meaning 'hear' or 'listen') is a statement of faith found in Deut. 6:4. It uses the plural word for God, Elohim, and the Hebrew word echad 'united/one/first' to describe His essence. (The word echad is used in Gen. 2:24 where two people, a husband and wife, equal in essence but separate in personhood, are united sexually as one but they are separate as people). Interestingly, the Shema does not use the Hebrew word yachid 'absolute one' to describe God which it easily could have if God wanted to drive home the point that He is numerically one. Surprisingly, this word is never used to describe the essence of God.
Humans are physical creations with a spiritual nature. God is a nonphysical Creator with a spiritual nature. Because God is Spirit by nature, His tri-unity is spiritual and not confined to physical limitations (John 4:23, 24). Humanity consists of different and separate human souls with separate personhoods. God consists of an identical and a united divine soul with separate Personhoods. Essence is what you are, Person is who you are. An essence is the soul that animates and motivates a person and gives them value. A person is the separate consciousness that delineates one being from another, giving them identity.
God cannot create others to be God in the way He is God (as they would have to be uncreated to be God). The true God is the only God that ever existed and the only one to ever be God. God is the only one worthy of worship and able to forgive sin, but in the Old Testament another Being does as well (Gen. 16:7-13; 31:11-13; Ex. 23:20-23; 34:6, 7; Is. 42:8; Dan. 7:13, 14)! By word and example, the Old Testament gives the possibility of a multi-personal, single God.
The New Testament Jew, who believed in one God (the Shema statement about God), worshipped Jesus after he proved his divinity (Matt. 14:28-33; 15:25; 28:17; John 20:28). The New Testament clarifies that the Father, Son, and Holy Spirit are divine and can forgive sin that only God alone can forgive (Mark 3:28-30; Luke 5:20, 21; 7:48, 49; John 14:17; Acts 5:3; Eph. 4:30). While billions have the separate souls of temporary humanity (us), only three share the soul/essence of eternal divinity (God).
The concept of the trinity or the 'tri-unity' of God (three persons united by one essence) is a biblical understanding of how God can become human and die for sin (Jesus) and dwell inside humans making them new (Holy Spirit), yet remain transcendent and separately God (the Father). There is none besides He and there will never be like Them!
Hindson, The Popular Encyclopedia of Apologetics