Roger Olson has an excellent post on the importance of defining doubt when one speaks of it in reference to faith.
Is doubt a necessary, even helpful aspect of Christian faith? Should faith conquer all doubt so that we regard as heroes of Christian faith those who seem to have risen above all doubt?
I think the answers to these questions must begin with definitions of "doubt." Much confusion is caused in Christian (as other) conversations by multiple (unstated) meanings of words.
Insofar as "doubt" indicates skepticism toward God, genuine unbelief, resistance to the submission of trust, I judge it to be always only a stage on the way to stronger faith and not an element of faith itself. This "doubt" is a disposition that resists trusting reliance on the truth of God and God's Word. This disposition is an indicator of the continuing liveliness of "the flesh" (as Paul calls the fallen human nature). It is a sign of need for greater submission to the transforming work of the Holy Spirit renewing the mind.
Insofar as "doubt" means lack of absolute certainty it is merely a sign of finitude. Similarly, insofar as "doubt" means partial understanding (of God and God's ways) it is merely a sign of finitude. I take it Paul is referring to these when he says that now we see in a glass dimly and only in the future will we see face-to-face. In this sense of "doubt" it is an element in faith because it constitutes admission of not-being-God. We are not capable, at least in this life, of "thinking God's thoughts after him." His ways are higher than our ways and his thoughts are higher than our thoughts. Admitting that is no sign of unbelief and stands in no tension with true faith.
Insofar as "doubt" means questioning and wrestling with notions about God we are told to believe but have trouble believing I judge it to be part of the process of "examined faith." We are instructed in the New Testament to "test all things" and "hold fast to that which is right." Questioning, examining, reflecting, thinking critically, using our God-given intellects to reason-- these can look like "doubting God" when they are only doubting human ideas about God with a disposition of wanting to believe and understand only what God has revealed.
I think it would be helpful if people would make clearer what "doubt" they mean when they talk about doubt as a positive aspect of the life of faith, of Christian living. Insofar as doubt spurs us on to greater dependence on God's revelation and faith and insofar as doubt causes us to question half-baked notions promoted by Christian communicators it is positive. Insofar as doubt constitutes a disposition of resistance to God's self-communication and dependence on him alone for self-understanding and understanding of answers to life’s ultimate questions communicated in God's Word it stands in tension with faith and is something to overcome with prayer: "Lord, I believe; help Thou my unbelief."
Roger's entire post can be read here.