Source : Webster's Revised Unabridged Dictionary (1913)

Believing \Be*liev"ing\, a.
   That believes; having belief. -- {Be*liev"ing*ly}, adv.

Source : WordNet®

believingly
     adv : in a credulous manner; "the children followed the teacher
           credulously" [syn: {credulously}] [ant: {incredulously},
            {incredulously}]