"The carnal mind is enmity against God for it is not subject to the law of God, neither indeed can be" (Romans 8:7).
Every normal person can determine what he will think about. Of course the troubled or tempted man may find his thoughts somewhat difficult to control and even while he is concentrating upon a worthy object, wild and fugitive thoughts may play over his mind like heat lightening on a summer evening. These are likely to be more bothersome than harmful and in the long run do not make much difference one way or another.
The best way to control our thoughts is to offer the mind to God in complete surrender. The Holy Spirit will accept it and take control of it immediately. Then it will be relatively easy to think on spiritual things, especially if we train our thought by long periods of daily prayer. Long practice in the art of mental prayer (that is, talking to God inwardly as we work or relax) will help to form the habit of holy thought.
This is what the Holy Spirit brings to us, the vision of the Lord, power to see divine things as God see them. The Spirit also thinks in us by giving us divine instincts, intuitions and enablements.
~A. W. Tozer~