The Practice of Virtue
Bernard of Clairvaux sees the practice of virtue as passing through three stages.
It begins at the level of disciplina. We have to learn from others what to do in order to be good. After many years virtue becomes habitual; it is second nature to us.
This is the level of natura. Finally, the actions that once cost us so much become not only dutiful or habitual but even gratifying.
This is the highest level, the level of gratia in which the human being is so transformed that godly acts are no longer a strain but a source of delight.
SC 23.6; SBOp 1.141-142