Possessed by a Demon
If a born-again believer is faithful to God, he can never be possessed by a demon. But if he decided, at a later time, to reject God and commit blasphemy against the Holy Spirit, he can certainly get possessed by a demon.
An example of this is King Saul. The Bible records that Saul not only was converted but that he also prophesied with the prophets: “And it happened, when all who knew him formerly saw that he indeed prophesied among the prophets, that the people said to one another, “What is this that has come upon the son of Kish? Is Saul also among the prophets?” Then a man from there answered and said, “But who is their father?” Therefore it became a proverb: “Is Saul also among the prophets?” And when he had finished prophesying, he went to the high place” (1 Samuel 10:11-13).
But later on, Saul chose to follow the path of pride. And he lost his connection with God in the process and committed the sin of blasphemy against the Holy Spirit (1 Samuel 13:14). Consequently, “the spirit of the LORD departed” from him (1 Samuel 16:14) and an evil spirit or demon afflicted him (1 Samuel 16:14).
It was not that the Spirit of God left Saul arbitrarily; but rather that King Saul rebelled against His leadership, and consciously withdrew himself from the protection of God. This must be understood in relation to Psalm 139:7 and with the principle of freedom of choice. God doesn’t force Himself on people. He respects their wishes and says, “choose this day whom you will serve” (Joshua 24:15).
As long as a person allows the Holy Spirit to teach him, to guide him, and to convict him, he is not guilty of committing the unpardonable sin. But if he refuses the Spirit’s ministry in his heart, then he starts the path of committing the unpardonable sin. Blasphemy against the Holy Spirit is therefore a constant resisting of the love of God’s Spirit to the point of not being able to hear His voice, thus hardening the conscience (1 Timothy 4:2).
This process is called the “grieving away” of the Holy Spirit. The Bible warns us “Grieve not the Holy Spirit, wherewith you are sealed” (Ephesians 4:30). Eventually, such a person loses the desire to repent, and therefore cannot be saved because he has rejected the Spirit that convicts of sin (John 16:8). When God’s spirit leaves, a demonic spirit takes hold of the heart and becomes the ruling power there.
In His service,