General antonyms:
* Praise
* Encouragement
* Commendation
* Approval
* Reward
* Affirmation
More specific antonyms:
* Indulgence (if chastisement implies harshness)
* Forgiveness (if chastisement implies punishment)
* Acceptance (if chastisement implies rejection)
* Understanding (if chastisement implies judgment)
The best antonym will depend on the context in which "chastisement" is used.