Before speaking to the damage of unbelief in God on the mind I first want to talk about the normal function of the mind. This is from the standpoint of what the Scripture tell us. And it will probably take more than one post.
The Scripture teaches that man is a tripartite being - spirit - soul - body.
"And the God of peace Himself sanctify you wholly, and may your spirit and soul and body be preserved complete, without blame, at the coming of our Lord Jesus Christ" (1 Thess. 5:23)
Man was created with three parts in order to contact three realms of reality. These are the three worlds of the spiritual, the psychological, and the physical.
The human spirit was created to contact the spiritual realm. The human soul was created to contact the ppsychological realm. And the human body was created to contact the physucal realm.
The five senses of the body are used to substantiate the physical world - hearing, seeing, smelling, tasting, and feeling.
Man can contact the spiritual realm with the human spirit which is distinct from the human soul. In our human spirit there is the sense of spiritual things and the ability to touch God, commune with God, and fellowship with God. Even the human spirit can receive God so that God and the believer become united and joined in life.
There is also the human mind designed by God to sunstantiate the psycholigical realm. The main part of the soul is the human mind.
The soul is for thinking
(Psa 13:2). So the Bible shows us that the mind is in the human soul.
The soul is for knowing
(Psa. 139:14), also proving that the Bible teaches that the mind is in the soul.
The mind also has an important place to play in things pertaining to the spiritual realm. Although it is the spirit of man which touches and contacts God directly, the Spirit of God witnesses with the human spirit that a life relationship with God has commenced upon believing in Christ:
"The Spirit Himself witnesses with our spirit that we are children of God. And if children ... joint heirs with Christ" (Rom. 8:16,17)
The spirit which God created in man
(Zech 12:1) can directly contact and commune with God the Holy Spirit
(John 4:24)
The human spirit can not only contact God the Spirit but also be made on in a joining of spirits which is an organic union between God and man -
"He who is joined to the Lord is one spirit" (1 Cor. 6:17)
The mind is needed to understand spiritual things. The mind is needed to remember the innermost sensation of contacting God the Spirit. The mind is needed to direct the rest of the human being to remain, abide, and linger in this realm of the joining of oneself to God the Spirit.