Well, I've found some citations, so now I have an answer: Samuel Clemens. This is an accepted Master's thesis that does a good job of explaining Clemens' lack of religion.
...perhaps the incident in Huckleberry Finn where the oversized Bible, spread on the chest of the dying Boggs, crushing the life out of him, is as symbolic of Twain's religious attitude as anything he ever wrote.
And this is a collection of his quotes. Here are a few of them:
[The Bible is] a mass of fables and traditions, mere mythology.
Man is a marvelous curiosity ... he thinks he is the Creator's pet ... he even believes the Creator loves him; has a passion for him; sits up nights to admire him; yes and watch over him and keep him out of trouble. He prays to him and thinks He listens. Isn't it a quaint idea.
There is no other life; life itself is only a vision and a dream for nothing exists but space and you. If there was an all-powerful God, he would have made all good, and no bad.
Note that the second link calls Clemens a man who believed in God but was not religious - this is patently false, as shown by many of the quotes. The issue with Clemens is that his actual, unedited Autobiography wasn't published until just recently, and even then, some material is being suppressed on his direct posthumous wishes.
EDIT - as noted in the comment, I only supply atheistic quotes and should in completeness supply Christian quotes. In fairness, I needed to find Christian and Scriptural references that Clemens did NOT use facetiously or ironically. This is difficult (without my rereading his canon), but I did find this one:
God puts something good and loveable in every man His hands create.