Indefectible

In′de‐fect″i‐ble (?), a. [Pref. in- not + defectible: cf. F. indéfectible.] Not defectible; unfailing; not liable to defect, failure, or decay.

An indefectible treasure in the heavens. Barrow.

A state of indefectible virtue and happiness. S. Clarke.