There are lots of different ways to be a theist. You could be a Christian, a Jew, a Muslim, a Hindu, etc. etc. And there are lots of different ways to be a Christian. Some ways of being a Christian (Unitarian perhaps, or UCC) have more in common, philosophically, with secular humanism than they do with other forms of Christianity like Evangelicalism.
By the same token there are lots of different ways of being an atheist. Ayn Rand and Josef Stalin were atheists, but then again so were Helen Keller and Carl Sagan.
People find their way to various forms of theism or atheism for all different kinds of reasons, and those reasons will have more to do with the rest of their worldview than where they land on the theism/atheism question.
Finally, all this ignores other ways of conceiving of the nature of faith and of God. According to Christian theologian Paul Tillich, faith is the act of being ultimately concerned. It's not a question of belief. He's very clear that the two should not be confused. Faith is an existential and not an empirical stance. Tillich thinks everyone will have faith in something, maybe it will be God, maybe Truth, maybe the Nation, maybe money or fame or something else. Most forms of faith are idolatrous in the sense that they cannot deliver ultimate fulfillment, though they require ultimate surrender.
This is what Hart fails to understand. When you look at faith this way, you can see that there are plenty of idolatrous theists and atheists, and plenty of theists and atheists with genuine faith. The atrocities he lays at the feet of atheism properly belong to secular forms of idolatry like communism and nationalism. Just being an atheist doesn't make your faith idolatrous, and just being a theist doesn't make your faith genuine.
(As for agnosticism, you can't really be an agnostic without taking for granted a key presupposition of most theistic faiths: that God's existence cannot be proven or disproven empirically. There is no evidence for this assertion. Indeed it's nothing more than a restatement of the definition of God. So ultimately the agnostic's position is just begging the question.)