Here the 1837 edition changed the definite article the to the indefinite article a in the phrase “as to the temporal death”, probably because of the surrounding occurrences of “a (spiritual) death” in this passage: “a death even a second death which is a spiritual death … shall also die a spiritual death”. Everyone suffers temporal death; thus the definite article the works perfectly well with temporal death. On the other hand, not everyone will suffer spiritual death; thus a works well with spiritual death. The critical text will restore the original definite article the before temporal death.
Lyle Fletcher (personal communication, 14 January 2004) suggests that the single occurrence of “then is a time” in verse 16 may be a mistake for “then is the time”, the phraseology that occurs twice in the following verse. The previous occurrence in verse 16 of three instances of the indefinite article a could have led to the replacement of an original the before time with a—just like “the temporal death” was changed to “a temporal death” in the 1837 edition. Nonetheless, one could consider the initial occurrence of time as the first reference to the time of spiritual death (thus the use of the indefinite article in “then is a time”), while the following two examples of “then is the time” refer to this same time of spiritual death (thus the use of the definite article). Here is another example involving the word time that shows the shift from the indefinite article a to the definite determiners this and the:
Thus the occurrence of “then is a time” is quite appropriate when spiritual death is initially mentioned in Alma 12:16–17.
Summary: Restore the original the that occurred before temporal death in Alma 12:16; also maintain in that verse the indefinite article a before time (“then is a time”) since this is the first reference to the time of spiritual death in this passage; subsequent references in verse 17 to this time use the definite article the (two occurrences of “then is the time”).