What you think about is who you are and what you are becoming. Your secret desires determine character. Rejecting sin starts in rejecting sinful thoughts. The mind is the arena where your greatest victories are won. Unless we let God’s Spirit shape us from the inside out, we remain shaped by our DNA, culture and our own choices. “Making every thought captive to the obedience of Christ” (2 Cor 10:5) requires spiritual discipline and constant prayer.
“Almighty God, unto whom all hearts be open, all desires known, and from whom no secrets are hid: cleanse the thoughts of our hearts by the inspiration of the Holy Spirit, that we may perfectly love thee, and worthily magnify thy holy name, Through Jesus Christ our Lord, Amen.” (From the Book of Common Prayer and the favorite prayer of John Wesley)