Few realize that it is a duty to exercise control over the thoughts and imaginations. It is difficult to keep the undisciplined mind fixed upon profitable subjects. But if the thoughts are not properly employed, religion cannot flourish in the soul. The mind must be preoccupied with sacred and eternal things, or it will cherish trifling and superficial thoughts. Both the intellectual and the moral powers must be disciplined, and they will ... improve by exercise.
In order to understand this matter aright, we must remember that our hearts are naturally depraved, and we are unable of ourselves to pursue a right course. It is only by the grace of God, combined with the most earnest effort on our part, that we can gain the victory. The intellect, as well as the heart, must be consecrated to the service of God. He has claims upon all there is of us.
Few believe that humanity has sunk so low as it has or that it is so thoroughly bad, so desperately opposed to God, as it is.... When the mind is not under the direct influence of the Spirit of God, Satan can mold it as he chooses. All the rational powers which he controls he will carnalize. He is directly opposed to God in his tastes, views, preferences, likes and dislikes, choice of things and pursuits; there is no relish for what God loves or approves, but a delight in those things which He despises....
If Christ is abiding in the heart, He will be in all our thoughts. Our deepest thoughts will be of Him, His love, His purity. He will fill all the chambers of the mind. Our affections will center about Jesus. All our hopes and expectations will be associated with Him. To live the life we now live by faith in the Son of God, looking forward to and loving His appearing, will be the soul’s highest joy. He will be the crown of our rejoicing.
Those who have trained the mind to delight in spiritual exercises are the ones who can be translated and not be overwhelmed with the purity and transcendent glory of heaven. HP 163
The hour and place of prayer are sacred, because God is there; and as reverence is manifested in attitude and demeanor, the feeling that inspires it will be deepened. Pr 209-210
"Search me, O God, and know my heart: try me, and know my thoughts: And see if there be any wicked way in me, and lead me in the way everlasting." Psalm 139:23-24 (King James Version)