As he (a man) thinks in his heart, so is he. Proverbs 23:7
A. P. Gibbs used to say, “You are not what you think you are, but what you think–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 your heart with all diligence, for out of it spring 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 steadfast spirit within me” (Ps. 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.
–William MacDonald, One Day at a Time, p.103