General:
* Expiation: This is the most direct synonym, meaning the act of making amends for a wrong.
* Redemption: This implies a freeing from guilt or punishment, often through sacrifice.
* Reconciliation: This emphasizes the restoration of a relationship.
* Reparation: This focuses on making amends for a wrong by providing compensation.
* Penance: This emphasizes self-punishment as a way to atone.
Religious contexts:
* Sacrifice: This refers to the act of offering something valuable to appease a deity.
* Propitiation: This emphasizes appeasing an offended deity.
* Salvation: This implies being saved from sin and its consequences.
More informal:
* Making amends: This is a straightforward and informal way to say "atoning".
* Paying one's dues: This implies that someone is making amends for past actions.
The best synonym will depend on the specific meaning you are trying to convey.