Theodore Epp emphasizes the significance of a renewed mind in his sermon, urging listeners to analyze their thoughts and recognize the impact of both good and bad thinking on their character. He explains that our thoughts shape who we are, as the heart reveals itself through our words and actions. Epp encourages the congregation to focus on spiritual and positive thoughts to build a strong character, while warning against the dangers of negative thinking that leads to deterioration. He highlights the transformative power of renewing the mind as a means to align with God's will.