Some assume that since Ellen White said the Holy Spirit represents Christ that it must be another being. And others believe that if the Holy Spirit is not a literal third being then there cannot be three.
But this wrong mindset comes from a lack of understanding of the power and capabilities of the Holy Spirit, and that the Holy Spirit is the ONE Spirit of the Father and Son.
“The HOLY SPIRIT is the SPIRIT OF GOD; it is also the SPIRIT OF CHRIST. It is that divine, mysterious emanation through which they carry forward their great and infinite work.” — (Uriah Smith, GC Bulletin, March 18, 1891, pp. 146, 147)
Thus 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. 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 beloved Son.
So whose Spirit represents Christ? Below Ellen White states that Christ has left HIS SPIRIT in the world to represent Himself.
Most respond, “How can His own Spirit represent Himself?”
Quite easily in fact, as Christ's Spirit can function independently of Himself like it is another. Many fail to realize that the Spirit of Christ can function like a person separate to Himself, so it is like it is “another” person.
This is the nature of the Holy Spirit that Ellen White said we cannot understand. So the Holy Spirit is indeed like a third person, though not a literal person.
“CHRIST has left HIS Holy Spirit to be HIS representative in the world, to give celestial aid to every hungering, thirsting soul.” — (E.G. White, Lt84, Oct 22, 1895)
Ellen White understood that “representative” does not make a third being, but that Christ represents Himself in a different form. God and His Son have a Spirit and that is how “They” represent “Themselves” where they are not personally present.
She made this very clear several times as we find from the following quotes.
“Cumbered with humanity, CHRIST could not be in every place personally; therefore it was altogether for their advantage that He should leave them, go to His father, and send the Holy Spirit to be His successor on earth. The Holy Spirit is [Christ] HIMSELF, divested of the personality of humanity, and independent thereof. HE would represent Himself as present in all places by HIS Holy Spirit, as the Omnipresent.” — (E.G. White, Lt119, Feb 18, 1895)
“The Holy Spirit is the SPIRIT of CHRIST; it is HIS representative. Here is the divine agency that carries conviction to hearts. When the power of His Spirit is revealed through the servants of God, we behold divinity flashing through humanity.” — (E.G. White, 13MR 313.3, 1895)
“CHRIST came to our world, but the world could not endure His purity. He has gone to His Father, but HE has sent HIS Holy Spirit to represent HIM in the world till he shall come again.” — (E.G. White, Ms1, Jan 11, 1897)
Thus we find that the Holy Spirit is the Spirit of God coming to us through His Son, and that Christ sent His Spirit to represent Himself. It is not another being as the Trinity doctrine claims.