Battle fatigue is real. But you have to remember that you're a soldier, like Flynn said. The timing for expressing the doubt you naturally feel is not while you're in the middle of a battle, in the trenches. Express the doubt you FELT after the battle is over and won. Courage is needed during the battle, and so good soldiers foster that in other soldiers.
Doubt in battle kills. Soldiers know that you still have to fight on, even if you feel doubt. Keep the faith!
It's a sin during this moment because you need as much faith as possible to continue the fight in this big battle. When things look down or dire is when faith and courage are needed most. That's why people are jumping on any doubt. We need to stay focused. Remember your faith and you'll be back in the fight like a good soldier.