i didn't say the "the" idea of god undermines ethics, only that "their" idea of god undermines ethics.
there is more than one way of coming at this, but the main one is: a religion wherein you have to behave a certain way out of fear of hell or hope of heaven actually undermines any notion that you do what is right because it is the right thing to do, rather than for a reward or to avoid punishment.* this is pretty much straight kant, except that i would say you don't have to buy into the whole kantian deontological ethics in order to accept and make use of this argument.
of course, kant also thought that while one ought to act out of respect for the rule rather than out of selfish motives, it is only right that people who DO act out of respect for the rule be rewarded for it, and so there must, after all, be a god to reward them.
sorry, that's a few more than 25 words, but i tried to adhere to the spirit of the request.
j
*in arguments, theists also like to allude to MLK to the effect that character (= ethics) is what you do when no one's watching, but in fact, for such people, god is *always* watching, and so there is no opportunity to demonstrate character (i.e., to be ethical). i have encountered this kind of argument in class more than once.
-- Among medieval and modern philosophers, anxious to establish the religious significance of God, an unfortunate habit has prevailed of paying to Him metaphysical compliments.
- Alfred North Whitehead