I do not doubt whether

WB forgive me

Just testing for possible software glitch

1234

Thanks WB for tolerating me

"If" is conditional. "I do not doubt/ he does not doubt" are definitive. "That" should be used in these instances.

"Whether" refers to alternatives. It's used in questions concerning doubt.
"Do you doubt whether...?"

Thanks for sharing.

Is your first language American English?

Yes.

Thank you.