Synonyms for Surely:

adv.

absolutely (adverb)
completely, certainly, supremely, Consummately, indubitably, positively, conclusively, categorically, utterly, unconditionally, definitely, unqualifiedly, unequivocally, unrestrictedly, perfectly, absolutely.
affirmatively (adverb)
Declaratively, assertively, Affirmatively.
categorically (adverb)
flatly, denominationally, typically, undeniably.
certainly (adverb)
naturally, incontrovertibly, unquestionably, irrefutably, steadfastly, undoubtedly, obviously, clearly, irrefragably, Doubtlessly, plainly, indisputably, incontestably, veritably, trustingly, assuredly.
indeed (adverb)
naturally.
really (adverb)
nothing else but.
safely (adverb)
Inviolably, safely, securely.
trustingly (adverb)
confidently, faithfully.
without doubt (adverb)
certainly, clearly, indubitably, assuredly, absolutely, distinctly, unerringly, evidently, fixedly, positively, unfailingly, rain or shine, explicitly, with certainty, admittedly, undoubtedly, conclusively, unequivocally, inevitably, Doubtlessly, nothing else but, manifestly, Infallibly, unmistakably, to be sure, definitely, unshakably, decidedly, for certain, plainly, irrefutably, unquestionably, beyond doubt.

adv

absolutely (adverb)
conclusively, unqualifiedly, completely, absolutely, definitely, supremely, utterly, certainly, unconditionally, Doubtlessly, unequivocally, unrestrictedly, perfectly, positively, Consummately, indubitably, categorically.
affirmatively (adverb)
categorically, positively, certainly, Affirmatively, definitely, Declaratively, assertively.
categorically (adverb)
categorically, denominationally, unequivocally, undeniably, indubitably, unqualifiedly, unconditionally, typically, certainly, definitely, flatly, absolutely.
certainly (adverb)
definitely, steadfastly, unquestionably, decidedly, absolutely, obviously, positively, trustingly, Doubtlessly, veritably, incontrovertibly, irrefutably, assuredly, undoubtedly, certainly, incontestably, irrefragably, unequivocally, clearly, indisputably, undeniably, indubitably, plainly.
hopefully (adverb)
trustingly, with confidence.
indeed (adverb)
to be sure, veritably.
positively (adverb)
with certainty.
really (adverb)
admittedly.
safely (adverb)
Inviolably, safely, securely.
trustingly (adverb)
trustingly, confidently, faithfully.
unavoidably (adverb)
inevitably.
unmistakably (adverb)
obviously.
without doubt (adverb)
certainly.

n.

essentially, fixedly, manifestly, unerringly, rain or shine, simply, admittedly, by all means, with certainty, unfailingly, with confidence, nothing else but, at any rate, distinctly, without doubt, evidently, precisely, beyond the shadow of a doubt, beyond doubt, explicitly, as a matter of course, at all events, with assurance, Infallibly, inevitably, without any doubt, unmistakably, in all conscience, most assuredly, unshakably, literally, actually, to be sure, beyond question, honestly, decidedly. funnily enough, Well, I never, of all things/people/places, is that a fact?, heavens above, who would have thought...?, you would not believe, now I've seen everything/it all, you don't say. of course, I don't mind if I do, yes, that's right, uh-huh, you're on, naturally. no doubt/question but that, beyond/without a shadow of a doubt, know for a fact (that), I bet/I'll bet (that), unless I'm mistaken, put money on something, correct me if I'm wrong, in my humble opinion. surely (noun)
certainly, for sure, sure as shooting, sure-enough, for certain, sure.

Other synonyms:

essentially, actually, evidently, honestly. literally, simply. precisely. clearly
distinctly.
surely
decidedly.
unavoidably
inevitably.
Other relevant words:
beyond question, by all means, inevitably, at any rate, actually, explicitly, to be sure, fixedly, for sure, precisely, unerringly, at all events, for certain, nothing else but, assuredly, sure as shooting, decidedly, sure-enough, manifestly, yes, most assuredly, naturally, Infallibly, unfailingly, sure, evidently, without doubt, distinctly, honestly, of course, uh-huh, unmistakably, beyond doubt.

Usage examples for surely

  1. Night would come as surely as anything did. – They Twinkled Like Jewels by Philip José Farmer
  2. Surely someone must have been before us. – The Golden Rock by Ernest Glanville
  3. Surely you must have seen her? – The Man Who Rose Again by Joseph Hocking