What is the Trinity?
Christian theology recognizes three separate aspects, or faces, of God: God the Father, God the Son (Jesus), and the Holy Spirit. "Trinity" is the word used to refer to these three aspects of God collectively. Strictly speaking, it is a theological term and not found in the Bible; however, the concept of the Trinity is found in the Bible. The clearest example can be seen at Jesus' baptism (Matt. 4:16), where we see God the Son standing in the river Jordan, the Holy Spirit descending in the form of a dove, and God the Father speaking from Heaven.
It's important to emphasize that this concept does NOT indicate three separate Gods, but rather three aspects of the one God. Many metaphors have been used to illustrate this idea (i.e., three different pages of the same book), but they are only loose approximations. In the end, we need to recognize that the Trinity is a mystery, something that is not fully understood and perhaps impossible for the human mind to completely grasp
"Who is he that overcometh the world, but he that believeth that Jesus is the Son of God? This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth. For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one." (1 John 5:5-7)
Notice at the end of this passage of Scripture that "the Father, the Word, and the Holy Ghost: and these three are one." is listed. This is the Trinity. One might ask, "Who or what is the "Word"? The term "Word" when translated from the Greek means the very essence, the very being thereof. John who authored the aforementioned passage of Scripture also uses this same wording in the first chapter of John's gospel.
"In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God. All things were made by him; and without him was not any thing made that was made." (John1:1-3)
The "Word" is Jesus Christ. Jesus is the very essence, the very being of God the Father. All three are distinct personages of the Godhead and yet all three constitute God. God the Father and Jesus the Christ who is His son share the same spirit. The spirit they share is referred to as the Holy Spirit.
All creation was made by means of Jesus. The very concept or thought that the creator of the universe would come to earth in the form of a man so that He could truly sympathize with our circumstances and later be a sacrifice to cover over for all the bad things each person has done was a theological bombshell in John's day. God wants a personal relationship with you. If you give Him your heart, He will give you His spirit. In addition to this, you will spend eternity with Him in Heaven one day.
"Hereby know we that we dwell in him, and he in us, because he hath given us of his Spirit." (I John 4:13)