I have hear it said by Rabbi's though I don't remember which ones, that a gentile doesn't do anything good without a motive behind it. An example would be giving charity. It is said that you should give charity because HaShem told you to. But gentiles (xians included) would only give charity with the ultimate understanding that they would in the end be blessed for doing it.

When it comes down to it I don't think anyone could do anything with out the understanding that what they do will reap something somewhere else, whether it is material or spiritual in the end.