I'm not sure that I had intended to convey that doing something nullifies it - meaning that they had it once but lost it, but rather meant to convey that for those who believe they must do something for it (in any way whatsoever for it), they, in believing so, at least to that point, have not yet received it. Further, just to clarify, should someone believe they will receive it because of their not doing anything for it, would, in effect, be the same as their doing something to try to receive it. Instead, I think it comes simply as realization, belief, and trust that Christ as the Saviour has already done everything on behalf of those He has chosen for salvation, and there is nothing remaining to be done by them - that it is fully and completely a gift. And they therefore happily find their spiritual rest in that. That realization and belief that He has done so, should someone truly have trust in it, can have only come to them as a result of being saved because the natural man of himself, will not/can not have it. There is nothing that anyone can do to save themselves as Christ alone is the Saviour.
Hope that makes sense.