"From the beginning Darwin was a creationist, and, despite the development of his theory, continued to be one. "

I find this claim questionable. While it is true that Darwin has tried to avoid the subject of religion / creation most of the time, his later works were much more critical of religion than his earlier ones. In the introduction of The Descent of Man, Darwin wrote:

"Ignorance more frequently begets confidence than does knowledge: it is those who know little, and not those who know much, who so positively assert that this or that problem will never be solved by science." (Charles Darwin, Introduction to "The Descent of Man," 1871)

Later on in the book he dismisses an argument for religion being innate:

"Belief in God- Religion.- There is no evidence that man was aboriginally endowed with the ennobling belief in the existence of an Omnipotent God. On the contrary there is ample evidence, derived not from hasty travellers, but from men who have long resided with savages, that numerous races have existed, and still exist, who have no idea of one or more gods, and who have no words in their languages to express such an idea. The question is of course wholly distinct from that higher one, whether there exists a Creator and Ruler of the universe; and this has been answered in the affirmative by some of the highest intellects that have ever existed."

"The belief in God has often been advanced as not only the greatest, but the most complete of all the distinctions between man and the lower animals. It is however impossible, as we have seen, to maintain that this belief is innate or instinctive in man. On the other hand a belief in all-pervading spiritual agencies seems to be universal; and apparently follows from a considerable advance in man's reason, and from a still greater advance in his faculties of imagination, curiosity and wonder. I am aware that the assumed instinctive belief in God has been used by many persons as an argument for His existence. But this is a rash argument, as we should thus be compelled to believe in the existence of many cruel and malignant spirits, only a little more powerful than man; for the belief in them is far more general than in a beneficent Deity. The idea of a universal and beneficent Creator does not seem to arise in the mind of man, until he has been elevated by long-continued culture."

His attacks on religion got sharper the older he became, and his posthumously published autobiography contained quotes about Christianity that were omitted by Darwin's wife Emma and his son Francis because they were deemed dangerous for Charles Darwin's reputation. Only in 1958 Darwin's granddaughter Nora Barlow published a revised version which contained the omissions. Here are some quotes which I have taken from the webpage <http://au.atheism.org/darwin.html>:

Evolving Disbelief

"Whilst on board the Beagle (October 1836-January 1839) I was quite orthodox, and I remember being heartily laughed at by several of the officers (though themselves orthodox) for quoting the Bible as an unanswerable authority on some point of morality. I suppose it was the novelty of the argument that amused them. But I had gradually come, by this time, to see that the Old Testament; from its manifestly false history of the world, with the Tower of Babel, the rainbow as a sign, etc., etc., and from its attributing to God the feelings of a revengeful tyrant, was no more to be trusted than the sacred books of the Hindoos, or the beliefs of any barbarian." (Charles Darwin: The Autobiography of Charles Darwin with original omissions restored. New York, Norton, 1969. p.85)

"By further reflecting that the clearest evidence would be requisite to make any sane man believe in the miracles by which Christianity is supported, --that the more we know of the fixed laws of nature the more incredible, do miracles become, --that the men at that time were ignorant and credulous to a degree almost incomprehensible by us, --that the Gospels cannot be proved to have been written simultaneously with the events, --that they differ in many important details, far too important as it seemed to me to be admitted as the usual inaccuracies of eyewitness; --by such reflections as these, which I give not as having the least novelty or value, but as they influenced me, I gradually came to disbelieve in Christianity as a divine revelation. The fact that many false religions have spread over large portions of the earth like wild-fire had some weight with me. Beautiful as is the morality of the New Testament, it can hardly be denied that its perfection depends in part on the interpretation which we now put on metaphors and allegories." p86

"Thus disbelief crept over me at a very slow rate, but at last was complete. The rate was so slow that I felt no distress, and have never since doubted even for a single second that my conclusion was correct." p.87

"I can indeed hardly see how anyone ought to wish Christianity to be true; for if so the plain language of the text seems to show that the men who do not believe, and this would include my Father, Brother and almost all my best friends, will be everlastingly punished. And this is a damnable doctrine." p87

The Design Argument

"The old argument of design in nature, as given by Paley, which formerly seemed to me so conclusive, fails, now that the law of natural selection had been discovered. We can no longer argue that, for instance, the beautiful hinge of a bivalve shell must have been made by an intelligent being, like the hinge of a door by man. There seems to be no more design in the variability of organic beings and in the action of natural selection, than in the course which the wind blows. Everything in nature is the result of fixed laws." p.87

Existence Of Suffering

"That there is much suffering in the world no one disputes. Some have attempted to explain this in reference to man by imagining that it serves for his moral improvement. But the number of men in the world is as nothing compared with that of all the other sentient beings, and these other suffer greatly without any moral improvement. A being so powerful and so full of knowledge as a God who could create the universe, is to our finite minds omnipotent and omniscient, and it revolts our understanding to suppose his benevolence is not unbounded, for what advantage can there be in the sufferings of millions of the lower animals throughout almost endless time? This very old argument from the existence of suffering against the existence of an intelligent first cause seems to me a strong one; whereas, as just remarked, the presence of much suffering agrees well with the view that all organic beings have developed through variation and natural selection." (emphasis added) p.90

The Existence Of God

"At the present day (ca. 1872) the most usual argument for the existence of an intelligent God is drawn from the deep inward conviction and feelings which are experienced by moat persons. But it cannot be doubted that Hindoos, Mahomadans and others might argue in the same manner and with equal force in favor of the existence of one God, or of many Gods, or as with the Buddists of no God...This argument would be a valid one if all men of all races had the same inward conviction of the existence of one God: but we know that this is very far from being the case. Therefore I cannot see that such inward convictions and feelings are of any weight as evidence of what really exists." p.91

"Nor must we overlook the probability of the constant inculcation in a belief in God on the minds of children producing so strong and perhaps as inherited effect on their brains not yet fully developed, that it would be as difficult for them to throw off their belief in God, as for a monkey to throw off its instinctive fear and hatred of a snake." p.93

Judging by Darwin's own statements, the only reason I see for him not becoming an outspoken atheist is the religious fundamentalism of his contemporaries.