The Trinity in the Bible represents the 3 forms of God: God, the Father; Jesus Christ, the Son; and, the Holy Spirit.

As stated in John 14, Jesus promised that the Holy Spirit would guide His followers once they accepted Jesus Christ as Lord of their lives. Because Jesus is "the way, the truth and the life" (John 14:6), the Holy Spirit is only made available after we come to know Jesus.

In the King James version, the Holy Spirit is also called the Holy Ghost and the Comforter. The NIV refers to the Holy Spirit as the Counselor.

Numerous instances of "ghost" exist in the King James version, especially when referring to deaths as "gave up the ghost".
The Old Testament has a few references to "Spirit of God" or "Spirit of the Lord" to set the stage, but most references come from the New Testament such as the many references in Acts to record the events of Pentecost when the Holy Spirit fell on the disciples.

A popular misconception is that the Holy Spirit represents our conscience. But, the Holy Spirit does much more than that when we try to stay in sync with God's will. Reading the Bible entirely is how we can distinguish between the lies of this world and God's ways.

The Holy Spirit was present in the lives of John the Baptist (see Luke 1:15 below) & Jesus Christ. After Jesus was baptized, the Holy Spirit came upon him. After his resurrection, Jesus gave the Holy Spirit to his followers on Pentacost.

God makes the Holy Spirit available to us when we accept Jesus as our personal savior. We must continuously follow God's guidance in the Bible to remain in sync with God.

As we allow the Holy Spirit to transform us, we should do our best to avoid ungodly actions called sins. Jesus states the sin that can NOT be forgiven: blasphemy of the Holy Spirit: Matthew 12:31-32, Mark 3:28-29, Luke 12:10 (passages below).