“It is not essential for us to be able to define just what the Holy Spirit is. Christ tells us that the Spirit is the Comforter, "the Spirit of truth, which proceedeth from the Father." It is plainly declared regarding the Holy Spirit that, in His work of guiding men into all truth, "He shall not speak of Himself." John 15:26 , 16:13, 52.” “The nature of the Holy Spirit is a mystery. Men cannot explain it, because the Lord has not revealed it to them. Men having fanciful views may bring together passages of Scripture and put a human construction on them, but the acceptance of these views will not strengthen the church. Regarding such mysteries, which are too deep for human understanding, silence is golden.” (Ellen G. White, ‘Acts of the Apostles’ pages 51-52, 1911)
A look at the original letter written to a certain Brother named Chapman from whence this quote from “The Acts of the Apostles” was taken reveals that Brother Chapman was erroneously teaching that the Holy Spirit was the angel Gabriel. That is, the Holy Spirit had angelic nature.
But remember, we are also told by inspiration that the Holy Spirit is Divine, or is “the Divine Spirit.” The Holy Spirit is certainly Divine in its nature as we understand the definition of the word “nature” in this sense.
But this meaning of the word “nature” as referred to the Holy Spirit (whether it is angelic or divine) cannot be the meaning that Mrs. White is referring to in her writings; otherwise its nature would not be a mystery.
To understand what Mrs. White means by saying that the nature of the Holy Spirit is a mystery, note again the very first sentence of the quote:
“It is not essential for us to be able to define just what the Holy Spirit is.”
Just what the Holy Spirit is, or its nature (what makes up the Holy Spirit), we do not know.
However, inspiration reveals that the Holy Spirit is the one Spirit that proceeds from God the Father. The Holy Spirit is the Father’s life, presence, and omnipresence.
The Holy Spirit has been given to Christ without measure by his Father.
The Holy Spirit also then proceeds from Christ to the human agent and therefore is also the personal presence of Christ with the believer.
The Father and his only begotten Son are both omnipresent and present with the individual believer by their one Spirit which they share.
Therefore, just what the Holy Spirit is, that is, the nature of the Holy Spirit which allows both Father and Son to be omnipresent and present with each individual believer is certainly a mystery, but as we just saw, the identity of the Holy Spirit of God is not a mystery.
I think that the following explanation is a summary of what is meant by the nature of the Holy Spirit being a mystery to us:
Just how a personal God, the Father, can be seated on the throne of His universe and how Jesus, a personal Son, can minister in the sanctuary above and yet both Father and Son be everywhere present by their Holy Spirit is the mystery not revealed to mortal minds.
In other words, what is the Holy Spirit that allows two personal Divine Beings to be omnipresent in the universe and present with each believer?
Answer: It is not essential for us to know only that when we receive the Spirit of God, we receive the Father and His Son and not a third Divine Being called God the Holy Spirit.