Jesus makes it plain that repentance includes changing our thoughts. "What comes out of a man, that defiles a man. For from within, out of the heart of men, proceed evil thoughts …" (Mark 7:20-21). He explains what defiles us comes from within. He gives examples of the most basic evil attitudes and passions that dominate our thinking and behavior: "… adulteries, fornications, murders, thefts, covetousness, wickedness, deceit, lewdness, an evil eye, blasphemy, pride, foolishness. All these evil things come from within and defile a man" (verses 21-23).
Isaiah states plainly that repentance is changing the way we think. "Let the wicked forsake his way, and the unrighteous man his thoughts; let him return to the Lord, and He will have mercy on him; and to our God, for He will abundantly pardon" (Isaiah 55:7). Here Isaiah pinpoints two things we must forsake to receive God's pardon: our sinful ways and our sinful thoughts.
To repent we must recognize that at least some of the pulls of human nature are at work in our minds, influencing or even controlling our thinking. Not every human constantly yields to every trait of human nature. But we all sin. We all have weaknesses. One person may succumb more often to greed, another to self-righteousness or pride. Yet another may have difficulty being truthful and honest. But in some way we all exhibit self-centered and self-serving thinking and behavior.
Repentance means we must examine ourselves and recognize our own particular weaknesses—the areas where we sin in thought or action. We must ask God to reveal what we need to change.
This, too, is a lifelong process. The more we submit to God and ask for His help in seeing what we need to change, the more He opens our minds to recognize our faults and weaknesses. This process goes on for years as committed and converted Christians "grow in the grace and knowledge of our Lord and Savior Jesus Christ" (2 Peter 3:18).
A change in the way we think is the most important aspect of real repentance. After we receive the Holy Spirit, we are better able to sustain right behavior as the fruit of our new way of thinking.