David, I gather that you're asking how this word made the transition from a reserve fund of merit to something that is unnecessary. well, it's through our old friend transferral. take a look at a more complete definition and it becomes a little more clear:

1. a. R. C. Theol. The performance of good works beyond what God commands or requires, which are held to constitute a store of merit which the Church may dispense to others to make up for their deficiencies.

b. transf. and gen. Performance of more than duty or circumstances require; doing more than is needed.


(so then Roget dumps it into a category called redundancy, and it gets lumped with superfluity :)