Discipline should be a trait of a disciple.
Discipline, as in self-control, is necessary to choose the shape of your own behavior. Without it, one cannot control their own choices, walk a path of repentance, nor follow the Lord.
Some would undoubtedly say, you can’t do any of those things anyway, that the human form is incapable of doing anything good. I agree the human form is frail, and incapable of leaving sin completely behind. We should reconcile ourselves to never being perfect.
But this fact does not mean we should not strive to do better, nor that we cannot do better. I distrust the definitions of Christian living which say either, our sinfulness glorifies God by allowing Him to give us more grace, or that our grace means sin is no longer relevant to us. Paul addresses both of these misconceptions in Romans 6.
Instead I view grace as most relevant in salvation. But discipleship, our growth after salvation is a partnership between us and God. Our obedience glorifies Him.
So consider the importance of discipline to the Christ follower.
Discipline is one of the benefits of fasting, discipline is necessary for the taming of the mind and body to the instructions of Christ, and discipline is required to replenish the storeroom of the heart with the things of God.