This quote is from a passage where Darwin is talking about religion:

At present the most usual argument for the existence of an intelligent God is drawn from deep inward conviction and feelings which are experienced by most persons. But it cannot be doubted that Hindoos, Mahomadans and others might argue in the same manner and with equal force in favour of the existence of one God, or of many Gods, or as with the Buddhists of no God....

Formerly I was led by feelings such as those just referred to, (although I do not think that the religious sentiment was ever strongly developed in me), to the firm conviction of the existence of God, and of the immortality of the soul... ...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 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....

Source: http://www.age-of-the-sage.org/faith_vs_reason_debate.html