One old and tired criticism of Christians is that we are 'only' mercenary. It asserts we only strive to do good so that we can sort of buy our way into Heaven while avoiding the punishment of Hell.
Setting aside the point that there's nothing terribly wrong with such an idea - do we actually want to teach that it's better to do evil if you're so inclined rather than do good from less than perfect convictions? - the criticism avoids the generally accepted fact that people are encouraged to be mercenary in other ways, for much less important goals.
Aren't we mercenary in pursuing a job or career path? To seek a nice house in a nice neighborhood? When on the athletic field? In finding a soul mate? While shopping for cool new things or deciding what to have for dinner? In all these areas and countless others aren't we seeking what is good for us, however mundane? Yet in looking for eternal joy we aren't supposed to consider what's good for us, what will make us truly happy?
Absurd. As a practical matter, I'm fine with someone not breaking into my house simply because he fears jail time. It would be better if he weren't to become a thief for the sake of higher ideals: respect for others and personal discipline for a start. Yet whether out of fear or a clear understanding and acceptance of moral good, he would have acted rightly either way.
Being merely mercenary can never condemn a man. It is less than perfect, yes. But if it slips us through the Pearly Gates and keeps the old homestead secure, I should think it worth the trade.
