One problem for standard theistic positions is the "Problem of Divine Hiddenness": if God wanted us to worship Him, why did He not make his presence more obvious?
The most popular reply, which I first encountered in Tim Keller's The Reason for God and is also trotted out in the Oxford University Philosophy of Religion course which is available on iTunes U, is that it gives us the option of choosing to love God. We can draw an analogy to a child who ought to love her sister. Her parents threaten to punish her if she does not love her sister. It seems sensible to think that even if the child does love her sister, if this love is motivated only by the threat of punishment then it is not love for the right reason. The child needs to make a choice to love her sister.
Take this back to the case of religion. For God to make his existence obvious, theist philosophers claim, would be equivalent to standing over all men with a fiery whip and proclaiming that men had better follow Him, or else! By giving us counter-evidence to His existence, God then secures for us the all-important good of freedom, freedom to choose to love Him.
It's a nice story, but it's one I find utterly unconvincing. In fact, when properly viewed this argument is in fact a very serious concession to non-theists.
Returning to the case of the sisters. Suppose the parents want their daughters to love each other, but do not want to force this. In order to achieve this, they first establish a pattern of only sometimes going through with threatened punishments. They then issue the original threat. There is therefore a degree of uncertainty, at least from the perspective of the child, as to whether the threat is genuine.
Suppose she decides to act as though it is. This is surely no different from the original case! In both scenarios she is motivated not by an intrinsic desire to love her sister, but by the threat of what her parents will do. The fact that the threat may not be entirely genuine is irrelevant.
Note, however, the concession made by the theist to get here. They had to argue that it would be wrong for God to give us strong or incontrovertible evidence for his existence. Consider Richard Swinburne's claim that "it is 97% probable that God exists." Is this really something that they want to be arguing?