How did Paul keep his body from becoming an instrument of unrighteousness?

Paul kept his body under control, exercising his will and refusing to yield his members as instruments of unrighteousness unto sin or old Adam. He kept old Adam under subjection rather than giving in, so his body would not become an instrument of unrighteousness. This required personal discipline, not self-torture.

Les explains that the constant admonition of Paul's letters to the believer is not to give in to old Adam but to keep him under subjection. Paul had to apply this in himself, and he says back in Corinthians that he kept his body under control, lest it became an instrument of unrighteousness. This is an exercise of will and yielding: do not yield your members as instruments of unrighteousness unto sin or old Adam. Les notes this does not mean becoming a cleric, monk, or torturing oneself; the Christian life is a life of freedom, joy, responsibility, and full enjoyment.

Why did Paul keep his body under control?

Paul kept his body under control lest it became an instrument of unrighteousness. The constant admonition in his letters is that believers should not yield their members as instruments of sin or give in to old Adam, but keep him under subjection. This is an exercise of will and yielding, not torture.