“The Holy Spirit cannot enter us until we have been purged and purified. That sometimes takes years after we are ‘saved’ when our heart of stone can be replaced with a heart of flesh.”
If you said cannot “fully posses us” it migt work, but Scripture is very clear that the Spirit enters into a soul at conversion, which is when they are washed, sanctified and justified (1 Cor. 6:11), even though, like the Corinthians, they lack degrees of maturity.
(Acts 2:38) “Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.”
(Acts 10:43-45) “To him give all the prophets witness, that through his name whosoever believeth in him shall receive remission of sins. {44} While Peter yet spake these words, the Holy Ghost fell on all them which heard the word. {45} And they of the circumcision which believed were astonished, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Ghost.”
(Acts 15:8-9) “And God, which knoweth the hearts, bare them witness, giving them the Holy Ghost, even as he did unto us; {9} And put no difference between us and them, purifying their hearts by faith.”
(Rom 8:9) “But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.”
(1 Cor 6:19) “What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own?”
(Gal 4:6) “And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father.”
There are degrees of maturity and we believe the same order is followed for everyone. First:
Matt 3:11 I indeed baptize you with water unto repentance, but He who is coming after me is mightier than I, whose sandals I am not worthy to carry. He will baptize you with the Holy Spirit and fireThis indicates that the baptism by fire is at the point of regeneration. This occurs further on as I'll explain below.
We understand "belief" as:
"Believing in the Lord is not just a matter of acknowledging Him, but includes keeping His commandments. Merely to acknowledge Him only demands some thought on the part of the understanding, but to keep His commandments requires acknowledgment too on the part of the will. The human mind is made up of the understanding and the will. It is the function of the understanding to think, of the will to act. So as long as a person's acknowledgment is merely in thought on the part of the understanding, he approaches the Lord with no more than half his mind; but when he acts, then he does so with his whole mind, and that is what believing is. " (TCR 151)
So right there this implies that we are to compel ourselves to repent our lives to make the commitment to the Lord and to cooperate with Him. The time this takes is up to the individual, but it's never instantaneous with conversion. Bad habits are hard to break.
"Those who believe in the Lord Jesus Christ are to receive those spiritual benefits, because He Himself is salvation and everlasting life. He is salvation, because He is the Saviour, for this is the meaning of His name Jesus; He is everlasting life, because those in whom He is, and who are in Him, have everlasting life. This is why He is called everlasting life in 1 John 5:20. Now since He is salvation and everlasting life, it follows that He is also every means which leads to salvation and everlasting life. Thus He is the whole of reformation, regeneration, renewal, quickening, sanctification and justification, cleansing from evils, and finally salvation. In the case of every single person the Lord confers these benefits, or rather, He attempts to impart them; and when a person makes himself ready and suitable to receive them, He does impart them. The activity of readying and making oneself suitable comes from the Lord too, but if the person does not receive them with spontaneity of spirit, then the Lord cannot go beyond the attempt to introduce them, and this attempt is constantly kept up." (TCR 150)
Note that salvation is at the end of the process. Again, some years may pass and the regeneration process lasts to eternity -- it's not a 'one time deal'.