Here the 1840 edition replaced assuredly as with as surely as. This change was also made in the 1852 LDS edition, probably independently (the 1840 edition was consulted to make corrections in the stereotyped plates for the second printing of the 1852 edition, but not for the first printing).
Elsewhere in the original text, there are four original occurrences of assuredly as, two of as surely as, and two of as sure as:
Here in Alma 37:45, the earliest textual sources (𝓞 is not extant here) spelled assuredly as with three words, as suredly as. This misspelling was probably the reason for the change to as surely as in the 1840 and 1852 editions. Basically, suredly was misread or misinterpreted as surely. For three of the four other examples of assuredly as, the earliest textual sources spelled the word assuredly without any space, thus avoiding in those three cases the tendency to replace the phrase assuredly as with as surely as. But the last example listed above, in Moroni 7:26, was spelled as suredly as in 𝓟 and in the 1830 edition, which led Joseph Smith in his editing for the 1837 edition to replace as suredly as with as sure as. This emendation was undoubtedly prompted by the original spelling as suredly as. The more colloquial as sure as was emended to as surely as in the 1981 LDS edition (but the two other instances of as sure as, in Alma 19:13 and Alma 23:6, were not). For further discussion, see under Moroni 7:26.
These eight other examples all involve an oath, either in reference to the Lord (six times) or in addressing a queen or king of the Lamanites (in Alma 19:13 and in Alma 22:8). The example here in Alma 37:45 is therefore different. The eight other examples indicate that in theory the text allows a free choice between assuredly as, as surely as, and as sure as. The critical text will therefore restore in Alma 37:45 the original phraseology, assuredly as (and similarly in Moroni 7:26).
Historically, the expression assuredly as was common in Early Modern English, as in the following examples found on and Literature Online ; typically, the expression occurs in the beginning of sentences (accidentals regularized here):
In these examples, assuredly as is equivalent to as surely as (or to as sure as, the more colloquial expression). This equivalence in expression can be found in today’s English, as in the following examples from of “just assuredly as” (with assuredly as preceded by just, as here in Alma 37:45):
The critical text will retain all instances in the Book of Mormon of assuredly as whenever they are supported by the earliest textual sources.
Summary: Restore the original assuredly as in Alma 37:45; the spelling of this phrase in the earliest sources as three words, as suredly as, led to the 1840 and 1852 misreading or misinterpretation of suredly as surely, giving the more standard expression as surely as; Moroni 7:26 contains another original example of the spelling as suredly as, which was emended by Joseph Smith in his editing for the 1837 edition to as sure as (and in later editing to as surely as).