saving grace 
noun [mass noun]
1The redeeming grace of God: the sudden intervention of God’s saving grace...
- That Sunday following the beginning of the war Reverend Isaac preached of saving grace.
- Izzy loves the Lord Jesus with all her heart and she wants to share the Lord with all who want to hear the good news about God's loving kindness and saving grace.
1.1 [count noun] A redeeming quality or characteristic: The car is hideously noisy with the hood up. Its one saving grace is that it is watertight...
- The sad reality, on a damp, windswept and cool weekend, though, was that the men's sprints are two of the saving graces of an otherwise disappointing championships.
- The first half was utterly forgettable with the exception of just a few saving graces.
- As rogues go, they are not very bad ones: they have their saving graces: and it would probably be impossible to muster a dozen honest men in politics to replace them.
Synonyms
redeeming feature, compensating feature, good point, thing in its/one's favour, appealing/attractive aspect, advantage, asset, selling point;
mitigating/extenuating feature