Of all the rational arguments against the God of the Bible, I think the most powerful is still that of the 18th century Scottish philosopher David Hume. As elegant as it is simple, the gist of his argument went like this: an all-powerful and supremely benevolent Creator should have made a better job of putting together a universe hospitable to humans than he appears to have done with this one. Not surprisingly, Hume's views on this subject (and on others besides) were so threatening to Scottish society that instead of making him a professor, the guys running the University of Edinburgh stuck him as far back in their library as they could and had him work some of its operations from there.
Atheists of more recent vintage are making out a lot better than Hume did. By way of examples, Sam Harris, Richard Dawkins, and Christopher Hitchens are selling like Starbucks in snow flurries, their products as overpriced as the coffee, while Hume's most important reflections on religion had to await publication until after his death. Just as Hume did, Harris, Dawkins, and Hitchens trade heavily on inept-inventor and failed-mechanic images of God. Hume, however, did it with subtlety, while these writers, Harris especially, come across as just plain grouchy. It seems to make them only mad, soooooo mad, that the Almighty may not have done his best by us. For Hume, the possibility that God learned enough from the experience to go on and make a better universe somewhere else was more a matter for humor, and even a little sadness, than it was for anger.
To give credit where credit is due, these new atheists are adding one other objection to the Bible's God that most reasonable people at least consider adding, even though it is one that Hume himself touched upon only briefly. The objection is that the world's allegedly great monotheistic religions have not made their followers actually better people. This is a hard fact to dispute, but the conclusion that many atheists often draw from it isn't. For them, the preponderance of sinners among the religious makes religion itself sinful and therefore worthy of being discarded. But staying away from church just because it is full of sinners makes no more sense than staying away from hospitals because there are sick people in them.
Paradoxically, the gap between believing in God and being godly both challenges and confirms monotheism's credibility. Facing up to our failures honestly can make Jews, Christians, and Muslims even more appreciative of a singularly important truth about human nature that the Old Testament, the New Testament, and the Qu'ran alike affirm as the central problem with which every religion sooner or later must deal. It is this: all --- the most religious, the least religious, and everyone in between --- fall short of the glory of God. In each of these faiths, the solution to the problem is the same: find in the scriptures the God who is truly worthy of being followed, separate Him/Her from the pack of lesser, deceiver gods that populate the pages of these same scriptures, and stop hoarding God's prerogatives for ourselves.
As Hume saw, and patiently expounded, and as Harris, Dawkins, and Hitchens see and impatiently declaim, there is much about the Bible that defies reason and good sense. Many of its passages are about a jealous, vindictive, tribal deity whose own actions eviscerate any authority he once might have had to demand perfection from anyone else. (You still think Abraham's willingness to sacrifice a son was a model for faith?) Even more speak of a divine governance that reaches down as far as the number of hairs on our heads, only to be ground down by modern science's clearly superior methodology for explaining how things work, even if the whys and the wherefores still fall within the provenance of faith. (You still think the facts which evolutionary biology explains are not facts at all?)
Maybe one of the reasons that the New Atheism is attracting so much attention is that so many are so fed up with religious right-wingers everywhere, for whom the words of their respective scriptures --- all of the words, and only those particular words --- constitute the last word about anything and everything on earth and in heaven. Reasonable Jews, Christians, and Muslims have always known better than to restrict the Word of God to the words of written texts and then to insulate those texts from further examination. Pursuing the examination is, of course, risky; it can even lead to ill-conceived atheism. But it is also one of the very best ways we have to love God with all of our minds.