If a person, who is denying the teaching of a Triune God, sincerely wants to have a Biblical faith, and be an authentic Christian without question, then this should be done. First, ask the Lord to show you wonderful truths from His Word. Second, read the Bible, alone. Start with Matthew 1:1 and continue through Revelation 22:21. Mark every verse that says something about the Holy Spirit. Write a summary statement as to what the text teaches, without reading anything into the verse.
When this is done, it will be impossible to disbelieve that God exists as Father, Son, and Holy Spirit.
The personhood of the Holy Spirit is affirmed by His personal pronoun. The Holy Spirit is referred to as a “He”, not as an “it”, or “thing.”
The personhood of the Holy Spirit is affirmed by the sins that can be committed against Him. we are not to grieve the Holy Spirit. “And grieve not the holy Spirit of God, whereby ye are sealed unto the day of redemption.” (Eph. 4:30)
The Holy Spirit can be sinned against. “But they rebelled, and vexed his holy Spirit: therefore he was turned to be their enemy, and he fought against them.” (Isaiah 63:10)
The Holy Spirit can be lied to. “But Peter said, Ananias, why hath Satan filled thine heart to lie to the Holy Ghost, and to keep back part of the price of the land?” (Acts 5:3)
Christians are to obey the Holy Spirit. “While Peter thought on the vision, the Spirit said unto him, Behold, three men seek thee. 20 Arise therefore, and get thee down, and go with them, doubting nothing: for I have sent them. 21 Then Peter went down to the men which were sent unto him from Cornelius; and said, Behold, I am he whom ye seek: what is the cause wherefore ye are come?” (Acts 10:19-21)
Christians are to honor the Holy Spirit. “Cast me not away from thy presence; and take not thy holy spirit from me.” (Psalms 51:11) A force, or power, cannot be sinned against. Only God can be sinned against.
The personhood of the Holy Spirit is affirmed by His equality with the Father and the Son. He too possesses the attributes of omniscience, omnipresence, and eternality. “Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: 20 Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen.” (Matt. 28:19-20)
The personhood of the Holy Spirit is affirmed by His many works.
The Holy Spirit was personally involved in creation. “And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.” (Gen. 1:2)
The Holy Spirit empowers God’s people. “Then he answered and spake unto me, saying, This is the word of the LORD unto Zerubbabel, saying, Not by might, nor by power, but by my spirit, saith the LORD of hosts.” (Zech. 4:6)
The Holy Spirit guides. “For as many as are led by the Spirit of God, they are the sons of God.” (Rom. 8:14)
The Holy Spirit comforts. “But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.” (John 14:26)
The Holy Spirit convicts. “And when he is come, he will reprove the world of sin, and of righteousness, and of judgment.” (John 16:8)
The Holy Spirit teaches. “Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come.” (John 16:13)
The Holy Spirit restrains sin. “So shall they fear the name of the LORD from the west, and his glory from the rising of the sun. When the enemy shall come in like a flood, the Spirit of the LORD shall lift up a standard against him.” (Isaiah 59:19)
The Holy Spirit gives commands. “Then the Spirit said unto Philip, Go near, and join thyself to this chariot.” (Acts 8:29)
Each of these works requires the involvement of a person rather than a mere force, thing, or idea.
The personhood of the Holy Spirit is affirmed by His personality.
The Holy Spirit has life. “For the law of the Spirit of life in Christ Jesus hath made me free from the law of sin and death.” (Rom. 8:2)
The Holy Spirit has a will. “But all these worketh that one and the selfsame Spirit, dividing to every man severally as he will.” (1 Cor. 12:11)
The Holy Spirit is omniscient. “But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God. 11 For what man knoweth the things of a man, save the spirit of man which is in him? even so the things of God knoweth no man, but the Spirit of God.” (1 Cor. 2:10-11)
The Holy Spirit is eternal. “How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God?” (Heb. 9:14)
The Holy Spirit is omnipresent. “Whither shall I go from thy spirit? or whither shall I flee from thy presence?” (Psalms 139:7)
A mere force could not possess all of these attributes, but the Holy Spirit does.
To deny the Holy Spirit as a person, and very God of very God, is to reject the Biblical doctrine of the Trinity, for there is no way to reconcile alternative views with the simplicity of Scripture.
The true Christian will say, “I believe in the Holy Spirit.” (The Apostle’s Creed, c. 390 AD)
The faithful Christian will declare, “That we worship one God in Trinity, and Trinity in Unity; Neither confounding the persons nor dividing the substance. For there is one person of the Father, another of the Son, and another of the Holy Spirit. But the Godhead of the Father, of the Son, and of the Holy Spirit is all one, the glory equal, the majesty coeternal. Such as the Father is, such is the Son, and such is the Holy Spirit.
The historic Christian will affirm, “I believe in the Holy Spirit, the Lord, the Giver of life, who proceeds from the Father and the Son, who with the Father and the Son is adored and glorified, who has spoken through the prophets.” (The Nicene Creed, 325 AD)
Let the Church be the Church. Let those who are not truly called by the Father, redeemed by the Son, and regenerated by the Holy Spirit leave the local body of Christ, and be honest enough to say, “I am an unbeliever.”