I think it's possible that this verse means that there is no mistake about God's calling or about the gifts he chooses to bestow. His gifts and callings are, in other words, irrevocable. There's no veto-ing what his will is, or what his calling is, there's only our free choice to ignore, contort, or reject it. 

I think this means that the Gifts and calling are from God even before we have repented of our sin. I know Mine sure was. Look at Paul, his calling was before he repented of his sin and he was one of the most influential of all his chosen.