The Holy Spirit is the mind, power and personal presence of the very life of God that the Father sends through His Son to us. Or to put it another way. The Holy Spirit is the presence and power of the Father manifested through Jesus Christ His only begotten son.
This is not a separate person of the Godhead who is being sent, it is the very life of God coming to us through His Son.
“The impartation of the SPIRIT is the impartation of the LIFE OF CHRIST.” — (E.G. White, DA, 805.3)
“JESUS imbues the believer with his spiritual life, WHICH IS THE HOLY GHOST.” — (E.G. White, 3SP, p. 242)
“CHRIST gives them the breath of HIS OWN SPIRIT, the life of His own life. The Holy Spirit [Christ] puts forth its highest energies to work in heart and mind.” — (E.G. White, DA, p. 827)
“The influence of THE HOLY SPIRIT IS THE LIFE OF CHRIST in the soul. We do not now see Christ and speak to Him, but HIS HOLY SPIRIT is just as near us in one place as another. It works in and through every one who receives Christ. Those who know the indwelling of the Spirit reveal the fruits of the Spirit,-love, joy, peace, longsuffering, gentleness, goodness, faith.” — (E.G. White, BEcho, June 17, 1901)
“WHAT WE WANT IS THE SPIRIT OF JESUS. When we have this, we shall love one another. Here are the credentials that we are to bear: “By this shall all men know that ye are my disciples, if ye have love one to another.” We need to pray more; and when we have Christ abiding in the soul, HIS SPIRIT IN ME will harmonize with HIS SPIRIT IN YOU; and he who controls our minds, controls also the heavenly intelligences, and they co-operate with us. Then in every council you will have the presence of One mighty in counsel. Jesus will be there.” — (Ellen G. White, GCDB, April 13, 1891)
When we grieve the Holy Spirit, whose Spirit do we grieve?
“O, how we grieve the pure, HOLY SPIRIT OF CHRIST with our defiling sins! We are not prepared for the appreciation of the holy communion with Christ and with one another unless we are cleansed by his efficacy.” — (E.G. White, RH, July 5, 1898)
“We can no more repent without the SPIRIT OF CHRIST to awaken the conscience than we can be pardoned without Christ.” — (Ellen G. White, SC, 26.2)
Our body is the temple of the Holy Spirit. So whose Spirit do we have abiding in us? “What? know you not that your body is the temple of the Holy Ghost which is in you,” 1 Corinthians 6:19
“When God’s people take the position that they are the temple of the Holy Ghost, CHRIST HIMSELF ABIDING WITHIN, they will so clearly reveal Him in spirit, words and actions, that there will be an unmistakable distinction between them and Satan’s followers.” — (E.G. White, Ms100, July 14, 1902)
Jesus told His disciples that after returning to His Father that He would come back to them by the Holy Spirit as their Comforter, “Do not let your hearts be troubled and do not be afraid. 28 You heard me say, 'I am going away and I am coming back to you.” John 14:27-28
Hence Jesus also said, “lo, I am with you alway, even unto the end of the world.” Matthew 28:20
How is Jesus going to be with us as our mediator, teacher as the Spirit of truth, Comforter etc. if He has returned to the Father? By coming back as another of the same kind. That is, through His Holy Spirit.
“While JESUS ministers in the sanctuary above, HE is still by HIS SPIRIT the minister of the church on earth. HE is withdrawn from the eye of sense, but HIS parting promise is fulfilled, “Lo, I am with you alway, even unto the end of the world.” Matthew 28:20. While HE delegates HIS power to inferior ministers, HIS energizing presence is still with HIS church.” — (E.G. White, DA, p. 166)
So who comforts and guides your life? The Spirit of God coming to us and manifested though His Son, hence the Spirit of Christ? Or some third being called “god the holy spirit?” The idea of a third being is a creation of Satan, brought into Christianity through the Catholic Church, so that he could be worshipped by unsuspecting deceived believers and steal their souls from the kingdom of heaven.