I've never read his journals or anything on his personal beliefs, which are in any case indeterminate--how do we genuinely verify someone's belief? Even the most devout question their faith at times, so our ability to definitely prove this is limited. Still, we can comment on some of the things that Darwin wrote and published. This is a substantial excerpt from his The Decent of Man (1871):
The moral nature of man has reached its present standard, partly through the advancement of his reasoning powers and consequently of a just public opinion, but especially from his sympathies having been rendered more tender and widely diffused through the effects of habit, example, instruction, and reflection. It is not improbable that after long practice virtuous tendencies may be inherited. With the more civilised races, the conviction of the existence of an all-seeing Deity has had a potent influence on the advance of morality. Ultimately man does not accept the praise or blame of his fellows as his sole guide though few escape this influence, but his habitual convictions, controlled by reason, afford him the safest rule. His conscience then becomes the supreme judge and monitor. Nevertheless the first foundation or origin of the moral sense lies in the social instincts, including sympathy; and these instincts no doubt were primarily gained, as in the case of the lower animals, through natural selection.
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....
Now, the highlighted bits don't prove that Darwin believed in God or even necessarily that he saw it as a good thing. But, it does appear to be the case that he saw it as reasonable to believe in an omniscient deity, and even an evolutionary advantage. It's of course possible that he's writing this with an ethnographic eye, attempting to explain the function of human societies while holding himself separate from them and the laws that seem to govern them. Still, there are key points at which he not only discusses why belief in God might be evolutionarily beneficial, but also makes implicit value judgments on it: an "elevated" culture produces belief in God, it helps the "advance" of morality, and is found among the "more" civilized people. Those words suggest that underlying this whole discussion is a baseline of ethnocentrism, in which his own society and its beliefs are the ultimate measuring stick for others.
So, this is hardly definitive, but just from what we have here my answer to "Did Charles Darwin believe in God" would be "sure."
When in Darwin's life are you asking about? His intention was once to become a member of the clergy, so he certainly believed in God at that point.
There's a famous quote available on Wikipedia from one of Darwin's letters:
With respect to the theological view of the question. This is always painful to me. I am bewildered. I had no intention to write atheistically. But I own that I cannot see as plainly as others do, and as I should wish to do, evidence of design and beneficence on all sides of us. There seems to me too much misery in the world. I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidæ with the express intention of their feeding within the living bodies of Caterpillars, or that a cat should play with mice. Not believing this, I see no necessity in the belief that the eye was expressly designed. On the other hand, I cannot anyhow be contented to view this wonderful universe, and especially the nature of man, and to conclude that everything is the result of brute force. I am inclined to look at everything as resulting from designed laws, with the details, whether good or bad, left to the working out of what we may call chance. Not that this notion at all satisfies me. I feel most deeply that the whole subject is too profound for the human intellect. A dog might as well speculate on the mind of Newton. Let each man hope and believe what he can. Certainly I agree with you that my views are not at all necessarily atheistical.
He's explicitly saying that his theory of evolution is not incompatible with an Enlightenment-style belief in God as a watchmaker, but it seems to me significant that he specifies the problem of evil, rather than his theory of evolution, as his main reason to doubt traditional Christian belief. Obviously I can only speculate regarding his thoughts, but I view him as a man who wished that a benevolent God existed, or even just that he could believe a benevolent God existed, but not as a man who believed in God.
Recently someone sold a letter from Darwin, at auction, which at least shows that he did not believe in Christianity. Given its ubiquity in his society you could say that this is tantamount to atheism but I'll leave the nuances to you: