John Calvin states that doubt is a part of the Christian faith.
“Surely, while we teach that faith ought to be certain and assured, we cannot imagine any certainty that is not tinged with doubt, or any assurance that is not assailed by some anxiety.”
John Calvin, Institutes of the Christian Religion. Book III, Ch 2.17 (Battles’ translation). Earlier, in Book III, Ch 2.4 he says “… unbelief is, in all men, always mixed with faith.”
HT: The Good book