The opposite of being disciplined is being lazy. There's too much of that going around, even among Christians today. Paul could have just coasted. After all, he was saved, right? Why did he work so hard to bring about the change in his life that God wanted? By that I mean why did he make the strong effort to cooperate with God the Holy Spirit in order to become the person God wanted him to become? The answer is much more simple than the process itself. Paul did this because he wanted to bring glory to God and he understood that whatever he was going through, it was because God ordained it and in every situation, he wanted to respond in a way that pleased God. Because of that, he literally disciplined himself - his thoughts, his actions and his words - so that he relied heavily on the power of the Holy Spirit to create in him (Paul), what was good for edifying others and glorifying God. This meant constant growth and change for Paul.