Is It a Sin to Doubt?
The Bible does not treat doubt as automatically sinful — Thomas questions the resurrection in John 20, and a father asks Jesus to "help my unbelief" in Mark 9:24 — and both are met with patience rather than rebuke.
Doubt shows up inside scripture itself
Thomas, John the Baptist from prison, and the psalmists all voice doubt directly to God rather than hiding it, and none are condemned for asking.
The difference between doubt and refusal
Most commentators distinguish honest doubt — wrestling with hard questions — from a settled refusal to believe regardless of evidence; scripture's patience is generally extended to the former.
Referenced
“Immediately the father of the child cried out with tears, "I believe. Help my unbelief!"”
Mark 9:24 · WEB
“Then he said to Thomas, "Reach here your finger, and see my hands. Reach here your hand, and put it into my side. Don't be unbelieving, but believing."”
John 20:27-29 · WEB