Self-mastery is self-government, self-control. A person who has self-mastery is a disciplined person, one who has his life in order. The first thing that a person has to accomplish when he begins to rid his life of evil is to get his own life in harness. Because, if we cannot control ourselves, we are sunk! The rest of the growth process will not follow. It all begins with mastering number one.
The thought in Ephesians 4 is that every part of the body has to contribute to the unity of the whole. How does one contribute to the church's unity? By making sure that one is growing, by letting grace teach us, by consciously responding to it.
Self-mastery can be even more strictly defined as "sober," which is how it is translated in the King James and New King James versions. This is a direct translation of the Greek word. In addition, it could also be translated as "sane"! Does this not indicate that before grace comes into our lives, we are all somewhat insane? Indeed, it does! Grace teaches us how to be sane—sound-minded! This is a looser translation, but it still fits.
This word also appears in Greek literature as "discreet," "self-discipline," "to behave in an orderly manner," "to be prudent," and "to be moderate," depending on the context.
We find it in the Bible in such places as Romans 12:3; Titus 2:6; and I Peter 4:7. It always suggests a person who is even-handed and has his passions under control, one who makes proper use of his drives and desires. This implication connects perfectly with the Christian work of getting rid of worldly passions. This word describes a person in whom there are no extremes in his or her manner of life. In the religious or spiritual sense, it means one who is making steady progress in growing into the balance of Jesus Christ. Someone who is not doing these things will be divisive because he is serving himself.