surely

doubtlessly, certainly, undoubtedly, fixedly, definitely, absolutely, evidently, explicitly, without doubt, beyond doubt, beyond question, plainly, infallibly, to be sure, most assuredly, unshakably, decidedly, inevitably, indisputably, positively, unquestionably, irrefutably, unfailingly, without any doubt, admittedly, clearly, with assurance, beyond the shadow of a doubt, nothing else but, precisely, conclusively, unequivocally, distinctly, by all means, in all conscience, at any rate, with certainty, unerringly, unmistakably, at all events, undeniably, manifestly, indubitably, with confidence, as a matter of course, rain or shine*.

Antonyms doubtful*, with no assurance, in doubt.