testify (v.)
late 14c., "give legal testimony, affirm the truth of, bear witness to;" of things, c.1400, "serve as evidence of," from Anglo-French testifier, from Latin testificari "bear witness, show, demonstrate," also "call to witness," from testis "a witness" (see testament) + root of facere "to make" (see factitious). Biblical sense of "openly profess one's faith and devotion" is attested from 1520s. Related: Testified; testifying; testification.