But I don't think doubt is all bad. Here is an edited version of my response to that friend.
Is doubt sinful?
Being discerning and not automatically believing everything someone tells you is not necessarily sin. For example, the Bereans in Acts 17 didn't just believe what Paul told them about Jesus. They listened and then they studied the Old Testament to make sure what he was saying about the Messiah could be true. Paul commends them for that kind of "doubt".
Also, "doubt" in the sense of having honest questions and confusion is not necessarily wrong either. I think these are actually good things IF we take them to God, the Bible, our fellow Christians, and pastors in hopes of finding answers. For example, John the Baptist preached for a long time that the Messiah was coming. When he baptized Jesus, John declared very clearly that He was the Lamb of God, the Messiah that he had been preaching about. But John didn't have all of the answers. He probably thought like most of the Jews that King Jesus was going to make everything immediately better. So when John ends up in prison, he gets quite confused. If Jesus really is the Messiah, then what in the world is going on? So in Luke 7, John sends messengers to Jesus to actually double check if He is the Messiah for real. John has a moment of doubt, but he takes those doubts to Jesus. Instead of Jesus chastising John for this, Jesus tells the messengers to go back and tell John that Jesus is fulfilling the prophecies of Isaiah. This is also the kind of "doubt" we see in the man who prays in Jesus' story and says, "I believe, help my unbelief". None of these things are shown as negative things in the Bible. On the contrary, they are shown as examples to follow. Taking your honest confusions and doubts to God for help is not a bad thing, but a good thing.
God works with doubters. Jesus answered John the Baptist, He didn't give Him the cold shoulder. Jesus commended the man who prayed for God to help his unbelief. Jesus even answered "doubting" Thomas by allowing him to have the proof he needed. All of these people want to believe, they just need a little help. God is willing to meet us in that.
The real danger with doubt, is when we become like the Pharisees. As I mentioned in Luke 7, there is this wonderful example of John wanting to believe and taking his very understandable doubts to Jesus looking for an answer. Also in Luke 7, is an example of sinful and wicked doubt. Just after getting done with John, Jesus says that there is a problem with most of this generation. They refused to believe John the Baptist. Why? Because John was a secluded weirdo who fasted and didn't eat bread or drink wine like a normal person. They refused to believe Jesus. Why? Because Jesus did eat bread and drink wine and hung out with normal people! They were simply making excuses. They didn't just have doubts and truly want to believe. They were unwilling to believe and made excuses out of everything. Its like nothing was good enough and when one sign was given, another was demanded. That is the sinful and dangerous kind of doubt we need to avoid.
What should we do when we doubt?
Young and old Christians need to be willing to bring their doubts and questions to God in prayer, pursue the answers sincerely in Scripture, seek help from pastors and friends, and pray for God to help their unbelief. Make an effort to believe and get around people who can help you.
Doubt is tricky when it comes to prayer. I don't think it takes a TON of faith for God to hear and answer a prayer. I see it kind of like a bank and money. If you give your money to TCF bank, how much does YOUR Faith in the bank matter? If you have rock solid faith in TCF, does that make the bank better able to protect and insure your money? Nope. If you have very little faith and worry and stress every day about the bank losing your money, is TCF any less able to protect your money? Nope. The amount of faith you have doesn't affect TCF in the slightest. BUT, has your faith or doubt affected your life? YES! It makes the difference between having a carefree and peaceful day or being worried all day long about the future.
In the same way, I don't think it takes a LOT of faith and MINIMAL doubt in order to see an answered prayer. Jesus said faith the size of a mustard seed could do amazing things. Its not about how much faith, but the object of your faith. A little faith put in TCF will cash out. A little faith put in God will cash out. The difference in little faith or lots of faith is not whether or not TCF or God will come through. The difference is whether you live your life trusting with peace in your heart and soul or doubting in anxiety and worry. All it takes is enough faith to actually offer up the prayer. The rest is up to God.
More faith honors God more and provides an easier life for you to be sure, but God can answer any prayer, even one offered with a lot of doubt.
So in short, my advice is this. Don't run from your doubts or be ashamed of your questions. Instead, take them to Jesus! Don't think for a second that your doubts cause you to be unreachable by God, know that He meets us where we are at. Beware of the attitude that says, "No matter what proof I see, I will not believe!" Humble yourself. Truly seek faith and turn to people that can help your unbelief when you come to those times in life where faith is hard.