General synonyms:
* Comforting
* Soothing
* Encouraging
* Calming
* Heartening
* Uplifting
* Hopeful
More specific synonyms:
* Convincing (if it's about dispelling doubts)
* Assuring (emphasizing certainty)
* Bolstering (if it's about increasing confidence)
* Strengthening (if it's about making someone feel more secure)
* Validating (if it's about confirming something)
The best synonym will depend on the specific situation and the desired tone.