Being honest, always costs something... or at least risks losing something.
If I tell him I think the drawing is terrible, he might never draw again.
If I tell her she's not the right person for the job, will she give up hope?
If I tell the soldier that we're hiding Jews, we all might die.
Yes, the potential costs vary widely but there is always a risk in being honest, which means there is always fear associated with honesty. This truth could affect the trajectory of my life, of his life, of her life. Which is why we're often dishonest, or at least we "sugar-coat" the truth. It's not worth the risk. (And sometimes it truly isn't.)
Factor in a belief in supernatural punishment for not being honest and the stakes get much higher.
If I admit my doubt, I might burn in hell forever.
I think this is why atheists and agnostics are often much more honest than fundamentalists. I think it's another of the reasons voices of God are always telling us to not be afraid. Honesty is worth it.