In Christianity, the Holy Spirit is the Spirit of GOD. In mainstream (Trinitarian) Christian beliefs he is the third person of the Trinity. As part of the godhead, the Holy Spirit is equal with GOD the Father and with GOD the Son. (both of the one substance (semen) from the godhead)
The Christian theology of the Holy Spirit was the last piece of Trinitarian theology to be fully developed. There is also greater diversity in Christian theology of the Spirit (pneumatology) than there is in the theology of the Son (Christology) or of the Father.
Within mainstream Christianity the Holy Spirit is one of the three persons of the Trinity. As such he is personal and also fully GOD, co-equal and co-eternal with GOD the Father and GOD the Son (godhead). He is different from the Father and the Son in that he proceeds from the Father (the substance (semen) of the father is restored unto the son) as described in the Nicene Creed” And in one Lord Jesus Christ, the Son of GOD, begotten of the Father [the only-begotten; that is, of the essence (semen) of the Father, GOD of GOD creator], Light of Light, very GOD of very GOD, begotten, (conceive) not made, being of one substance (semen) with the Father;. His sacredness is reflected in the New Testament gospels (e.g., Mark 3:28-30, Matthew 12:30-32, and Luke 12:8-10), which proclaim blasphemy against the Holy Spirit as unforgivable.
The Holy Spirit is believed to perform specific divine functions in the life of the Christian or the church. These include:
Conviction of sin. The Holy Spirit acts to convince the unredeemed person both of the sinfulness of their actions, and of their moral standing as sinners before GOD.
Bringing to conversion. The action of the Holy Spirit is seen as an essential part of the bringing of the person to the Christian faith. The new believer is "born again of the Spirit".
Enabling the Christian life. The Holy Spirit is believed to dwell (spirit within) in the individual believers and enable them to live a righteous and faithful life.
As a comforter or Paraclete, one who intercedes, or supports or acts as an advocate, particularly in times of trial.
Inspiration and interpretation of scripture. The Holy Spirit both inspires the writing of the scriptures and interprets them to the Christian and/or church.
The Holy Spirit is also believed to be active especially in the life of Jesus Christ, enabling Him/him to fulfil his good works on earth. Particular actions of the Holy Spirit include:
Cause of His/his birth (conception/creator/semen). According to the gospel accounts of the birth of Jesus, the "beginning of His/his incarnate existence", was “dew” the Holy Spirit and Holy Ghost. (semen)
Anointing him at his baptism.
Empowerment of His ministry. The ministry of Jesus following His baptism (in which the Holy Spirit is described in the gospels as "descending on Him like a dove") is conducted in the power and at the direction of the Holy Spirit.
Trinitarian and Non-Trinitarian Christians have different doctrinal views of the Holy Spirit.
The Holy Spirit is frequently referred to by metaphor and symbol, both doctrinally and biblically (Holy Spirit and Holy Ghost). Theologically speaking these symbols are a key to understanding of the Holy Spirit and His/his actions, and are not mere artistic representations.
Water - signifies the Holy Spirit's action in Baptism, such that in the manner that "by one Spirit [believers] were all baptized", so they are "made to drink of one Spirit". (1 Corinthians 12:13) Thus the Spirit is also personally the living water welling up from Christ crucified (John 19:34; 1 John 5:8) as its source (thy godhead) and welling up into Christians to eternal life.
Anointing - The symbolism of anointing with oil also signifies the Holy Spirit, to the point of becoming a synonym for the Holy Spirit. The Coming of the Spirit is referred to as his "anointing". (Cf. 1 John 2:20,27; 2 Corinthians 1:21) In some denominations anointing is practiced in Confirmation; ("chrismation" in the Eastern Churches). Its full force can be grasped only in relation to the primary anointing accomplished by the Holy Spirit, that of JESUS. Christ (in Hebrew, messiah) means the one "anointed" by GOD's Spirit. (Holy Ghost)
Fire - symbolizes the transforming energy of the Holy Spirit's actions. In the form of tongues "as of fire", the Holy Spirit rested on the disciples on the morning of Pentecost.
Cloud (Dew) and light - The Spirit Comes upon the Virgin Mary and "overshadows" her, so that she might conceive (Immaculate Conception) and give birth to JESUS. On the mountain of transfiguration, the Spirit in the "the cloud Came and overshadowed" JESUS, Moses and Elijah, Peter, James and John, and "a voice came out of the cloud, saying, 'This is my Son, my Chosen; listen to him!'" (Luke 9:34-35)