What is the Trinity?

The doctrine of the Trinity states that the Father, the Son, and the Holy Spirit coexist in the unity of one God. "Trinity" is never mentioned in the Bible. It is the Christian Church's way of reconciling the Old Testament teaching that there is only one God (Deuteronomy 6:4) with New Testament teachings about the Father, Son and Holy Spirit.

Trinity [is] the term by which is expressed the unity of three Persons in the one God. The Christian doctrine is: (1) That there is only one God, one divine nature and being. (2) This one divine Being is tripersonal, involving the distinctions of the Father, the Son, and the Holy Spirit. (3) These three are joint partakers of the same nature and majesty of God.
From New Unger's Bible Dictionary, Moody Press, 1988.

God the Father is the creator and sustainer of the universe. He is creator (Genesis 1:1), purely spiritual (John 4:24), all-powerful (Genesis 18:13-14). God is love (1 John 4:8-16).

Jesus, Son of God the Father, came to us in human form for our salvation (John 3:16, 1 John 4:8-12). Jesus is identified as "Son of God" about 40 times in the New Testament (Matthew 14:33, Mark 1:1, Luke 1:26-38, John 20:30-31, Galatians 2:20, etc.) There are several verses that identify Jesus more closely with God the Father (John 8:19, 10:30, 14:8-11). Jesus is no longer on earth in bodily form, but He is still with us in Spirit (Matthew 28:18-20).

Many people think "Christ" was Jesus' last name, but "Christ" is actually a title. "Christ" is the Greek equivalent of the Hebrew word "Messiah," which means "anointed one." There were many prophecies in the Old Testament of a coming Messiah who would be the savior of the Jewish people (Isaiah 11:1-9, 42:1-4, Jeremiah 23:5-6, Malachi 3:1, etc.) Those prophesies were fulfilled in Jesus (Matthew 1:1, 16:15-17, Luke 1:30-33, John 20:30-31, etc.) However, Jesus was not the powerful military and political leader many of the Jews had expected.

The Holy Spirit is the counselor, helper and comforter (Romans 5:5, John 15:26, 16:13-15). The Holy Spirit is, in a way, the emissary the Father and Son have sent to us. The Father and Son speak to us, guide us, and send us Their love via the Holy Spirit.

The original New Testament used the Greek term Pneúma tó Hágion. It literally means "the Holy Wind," and it conveyed the idea of the breath of God as invisible and immaterial but powerful. The old King James Version of the Bible usually translated this into English as "Holy Ghost." However, the word "ghost" has now taken on the meaning of a disembodied soul, and modern Bible versions use the term "Holy Spirit" instead.

The exact relationship between God the Father, Jesus the Son, and the Holy Spirit is never spelled out in the Bible, and it may well be beyond our human ability to understand. The doctrine of the Trinity is the Christian Church's attempt to reconcile the various Bible teachings and to understand this great spiritual mystery.