"And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness" (Romans 8:10).
What is the Holy Spirit? Not who, but what? The answer is that the Holy Spirit is a Being dwelling in another mode of existence. He has not weight, nor measure, nor size, nor any color, no extension in space, but He nevertheless exists as surely as you exist.
The Holy Spirit is not enthusiasm. I have found enthusiasm that hummed with excitement, and the Holy Spirit was nowhere to be found that at all; and I have found the Holy Spirit when there has not been much of what we call enthusiasm present.
Neither is the Holy Spirit another name for genius. We talk about the spirit of Beethoven and say, "This or that artist played with great spirit. He interpreted the spirit of the master." The Holy Spirit is none of these things. Now what is He?
He is a Person. Put that down in capital letters. He is Himself a Person, with all the qualities and powers of personality.
The Spirit is Himself God, the very true Nature of the Godhead subsisting in a form that can impart Himself to our consciousness.
~A. W. Tozer~