The technical name for the fallacy is "complex question." Imagine if someone were to ask you: "have you stopped beating your wife yet?" It can't be answered in yes or no fashion, which is what the foolish unbeliever most likely wants because he knows that either answer would trip you up. Like everyone else has said, the question rests on a faulty understanding of God's nature. The person asking it assumes that we believe that being God means being able to do anything but this is not the case.

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Davidius
Husband of Emily
Member of All Saints Anglican Church - Chapel Hill (AMiA / Anglican Church of North America)
Student: University of North Carolina at Chapel Hill, German and Classics