Willie Mullan emphasizes the doctrine of sanctification, explaining that it means to separate or set apart, and is a crucial teaching found throughout the Bible. He clarifies that sanctification is not synonymous with the second blessing, as many misunderstand, and illustrates this through various scripture references, including 1 Corinthians and John 10. Mullan shares personal experiences to highlight the transformative power of sanctification in a believer's life, emphasizing that it is a process that involves being set apart for God's purposes. He concludes by affirming that true sanctification is rooted in the sacrifice of Jesus Christ and is essential for every believer's journey.