See the "Descent of Man," vol. i. p. 68 (original edition). But it is right to state that the subject is not treated in any way whatever so as to argue that the religious belief is a fancy, or development of fancy, with no God and no facts about God behind it.
It is not necessary to my immediate argument, and therefore I do not press it into the text (though I should be sorry to seem to forget it for a moment), to urge that St. Paul draws a clear distinction between the intellectual faculties and the higher spiritual ones, when he assures us that the clearest intellect alone cannot assimilate the truths of religion. For the spiritual faculties have been in man grievously deadened and distorted (to say the least of it), so that his intellectual faculties, bright and highly developed as they may be, will always prove insufficient for the highest life in the absence of the "grace of God." It is exactly analogous to the case of a man whom we might suppose to have his sense of sight, touch, &c., distorted, and he himself unable to correct them by aid of the senses of others. However acutely he might exercise his reason, he would be continually wrong in his conclusions. See 1 Cor. ii., the whole, but specially vers. 14, 15.
"Descent of Man," vol. i. p, 70.