authoritatively

adv

1. In an authoritative manner; with authority.

   “He has, of course, the right to speak authoritatively, and he laughs at my theory that there was a crack in Pansay's head and a little bit of the Dark World came through and pressed him to death.”

   “The inability of Trump's own spokespeople to speak authoritatively on the president's thinking and actions -- a challenge even Trump himself has acknowledged -- has left the White House with an enormous credibility gap, and has tarnished the reputations of many of its most visible spokespeople.”