I have encountered people who say that faith and doubt not only can, but should co-exist, in the same person at the same time, and as to the same object. An acquaintance has gone so far as to assert that faith without doubt is inhuman, immature, and cowardly.
I reply that faith and doubt are incompatible.
To my mind, “doubt” is basically indecision: We have not yet determined whether to believe or assent to something. We don’t believe it’s true, and we don’t believe it’s false. If we believed it were true, we would no longer be doubting; and if we believed it were false, we would no longer be doubting; for in either case our mind would be made up.
To have faith is to decide to believe that something is true. Once we have so decided, we are no longer in a state of doubt. If doubt re-enters the picture, then we are no longer in a state of faith. It’s certainly possible to waver between belief and unbelief, but it’s not possible to do both at once.
Cardinal Newman writes that there are three types of propositions: Interrogative, conditional, and categorical. You may ask a question (interrogative); you may draw a conclusion (conditional, since it depends on premisses); or you may make an assertion (categorical). He writes further that these types of propositions correspond to three modes of holding propositions in the mind: Doubt (interrogative), inference (conditional), and assent (categorical).
Applying these three modes to revealed religion, Newman writes that a man is either a skeptic towards religion; a philosopher, having arrived at the conclusion that it is more or less probable based on logical inferences; or a believer, having an unhesitating faith in it.
You may alternate between these states at different times. Also you may infer and assent simultaneously; but you can’t infer and doubt, or assent and doubt, at the same time.
To put it another way: When you are in a state of doubt, you are questioning whether a thing is true. It is simply not possible to question and assent to the same proposition at the same time.