My answer is always, "It depends."
They ask because it's a common thing to hear in the church that doubt is forbidden. They've heard that the second doubt about God or the Bible pops in your head you need to crush it like a pre-teen boy hunting Whack-a-Moles. In some ways, I understand why this was taught. It's a lot easier to tell someone to just believe than to actually have to grapple with their thoughts. Or maybe they were afraid the doubt would transform into a foaming-at-the-mouth atheism. Better to squash it now. But the problem is when a genuine question about God comes along (Why would a loving God command killing all the Canaanites in the Old Testament? If Jesus was God, how could He die on the cross? What's a holy kiss and where can I get one?), it is construed as rebellion against the faith. Why can't you just believe?
Forbidding doubt will kill questions. Killing questions births a faith that doesn't know why it exists. They "just" believe. But when an intellectual argument against their faith comes, they are left in the street naked without answers. Or when a horrific tragedy strikes their lives, they have no anchor to keep them from drifting. They always just believed. Now they see their faith for what it was—hollow. This is why so many Christians go off to college, get hit with every ideology under the sun, and walk away from their faith.
We need a new way to think about doubt and questions; one that acknowledges the dangers doubt can pose but also the benefits doubt can give.
DOUBT CAN BE BENEFICIAL
Doubt that's beneficial is one that looks for answers—looks for truth. You know an answer is out there. You're not trying to undermine two thousand years worth of prayer, scholarship, and theology. You just want to know.
This doubt is extremely beneficial because Christians aren't supposed to "just" believe. The Christian faith is one that's grounded in evidence—in facts. The Gospel is not just a collection of truths but it's a description of a historical event. Real people in real places. There's an abundance of evidence to support our faith. We shouldn't be horrified of doubts, as if every time a young believer asks a question on hell an angel loses its wings. Instead, we should confidently answer their questions and help them through their doubts.
If you don't know the answers, figure them out yourself. If you're having doubts, answer the doubts with Scripture. We should know why we believe not just that we believe. "Always being prepared to make a defense to anyone who asks you for a reason for the hope that is in you" (1 Peter. 3:15). If someone asks you why you believe what you believe, would you be able to answer them?
Doubt can be helpful but only if you desire the truth. Don't doubt for doubt's sake. Wrestle with your doubt so you can know God. Doubt can be the seed of faith. If we truly want the truth, our faith will be stronger for it.
DOUBT CAN BE HARMFUL
Doubt that's harmful is one that looks for anything but the answer. The truth was given to you but you're unwilling to accept it. So you go looking for something else or believe there is no answer at all.
If we're truly seeking for truth, then we should be willing to accept the answer even if we don't like it. But we humans are fickle beings. "For the time is coming when people will not endure sound teaching, but having itching ears they will accumulate for themselves teachers to suit their own passions" (2 Timothy 4:3). They didn't like the truth because it didn't hit their sweet spot, so they went searching for cheap substitutes.
You don't like the idea that people who reject Christ go to hell? We'll work something out.
You don't like the idea that homosexuality (or any sexual expression outside traditional marriage) is a sin? I have just the thing for you.
You don't like the idea that being a disciple of Christ means you have to die to yourself and your desires? You can have your best life now.
This kind of doubt isn't a wrestling for truth. It's unbelief. A refusal to believe the truth you see before you. This kind of doubt doesn't strengthen faith—it undermines faith.
I do believe doubt and questions should have a place in the church. I love hearing people work through the Bible and wrestle with what it says. That means our brains are working. That means we're trying to figure out what it means to love the Lord with all our mind. But in the middle of our doubting, remember Christ is standing there waiting for us to touch the holes in His hands. We just have to be willing to see Him.