~ Hard of Hearing ~
In every internet debate I've ever had regarding religion (almost always with a Christian fundamentalist), I bring up the Euthyphro dilemma. Before Christianity even had its start, good ol' Plato cut it off at the knees with one sentential swipe. His reasoning goes something like this: if an action is moral only because a god says so, then morality is arbitrary; but if it's moral because it coincides with moral reality (what's objectively real), then morality is independent of a divine will (i.e., a god is good because it subscribes to the same reality that we mortals do). In either case, we don't need a god for morality. However, I've yet to meet a Christian who's convinced by this argument -- such is the function of faith -- but if he's intellectually inclined, he'll acknowledge that the argument is important enough to be dealt with. After all, what good is a religion that doesn't ground morality? Religions suck at doing science and are even worse at giving day-to-day practical advice. Thus, there has been a fine, honorable tradition of Christian rationalist attempts to explain away Plato's argument.