William MacDonald emphasizes that sin originates in the mind, asserting that our thoughts shape our actions and ultimately our character and destiny. He highlights the importance of controlling our thought life, as Solomon advises in Proverbs, and warns that harboring negative thoughts can lead to sinful actions. MacDonald illustrates this with Jesus' teachings, equating thoughts of hatred and lust with their corresponding sins. He encourages believers to take responsibility for their thoughts, suggesting prayer, self-examination, and filling the mind with positive influences as ways to cultivate a pure thought life. Ultimately, he reminds us that we have the power to rule our thoughts and direct them towards good or evil.

“…as (a man) thinketh in his heart, so is he.” (Prov. 23:7) A. P. Gibbs used to say, “You are not what you think you are, but what you think—this is what you are.” This means that the mind is the spring from which behavior flows. Control the source and you control the stream that flows from it. Therefore control of the thought-life is basic. That is why Solomon said, “Keep thy heart with all diligence; for out of it are the issues of life” (Prov. 4:23) Here the heart is used as a synonym for the mind. James reminds us that sin begins in the mind (Jas. 1:13-15). If we think about a thing long enough, eventually we’ll do it. Sow a thought and reap an act. Sow an act and reap a habit. Sow a habit and reap a character. Sow a character and reap a destiny. The Lord Jesus emphasized the importance of the thought-life by equating hatred with murder (Mt. 5:21, 22) and by equating the lustful look with adultery (Mt. 5:28). He also taught that it’s not what a man eats that defiles him but what he thinks (Mk. 7:14-23). We are responsible for what we think because we have the power to control it. We can think about lewd, suggestive things or we can think about what is pure and Christlike. Each one of us is like a king. The empire that we rule over is our thought-life. That empire has tremendous potential for good and enormous potential for evil. We are the ones who determine which it will be. Here are some positive suggestions as to what we can do. First, take the whole matter to the Lord in prayer, saying “Create in me a clean heart, O God; and renew a right spirit within me” (Psa. 51:10). Second, judge every thought by how it appears in the presence of Christ (2 Cor. 10:5). Third, confess every evil thought instantly and expel it (Prov. 28:13). Next, avoid having a blank, empty mind. Fill it with positive, worthy thoughts (Phil. 4:8). Fifth, exercise discipline over what you read, see, and hear. You cannot expect a pure thought life if you feed on filth and pollution. Finally, keep busy for the Lord. It’s when you shift your mind into neutral that vile fantasies seek admission.