adj. Experiencing or harboring no doubts; entirely confident.
The Century Dictionary and Cyclopedia
Not doubting; not hesitating respecting facts; not fluctuating in uncertainty: as, an undoubting believer; an undoubting faith.
Word Usage
"Brooke himself, Churchill assured readers of his obituary, had been "joyous, fearless, versatile, deeply instructed ... ruled by high undoubting purpose.""