Sometimes, Christians have doubt. That’s not sinful. The struggle for understanding and wisdom should be a part of our faith. It increases our capacity in knowledge. But, do keep this from Isaiah close in your range:

Isaiah 55:8-9
New International Version (NIV)

8 “For my thoughts are not your thoughts,
neither are your ways my ways,”
declares the Lord.
9 “As the heavens are higher than the earth,
so are my ways higher than your ways
and my thoughts than your thoughts.