the demands of divine justice doth awaken his immortal soul to a lively sense of his own guilt
Here Oliver Cowdery initially wrote repent in the printer’s manuscript; then almost immediately he added the eth (the level of ink flow for the supralinearly inserted eth is unchanged). The two following verbs in the if-clause also take the -(e)th ending (remaineth and dieth), so this scribal correction is consistent.
The question here is whether the verb in the if-clause should be in the indicative (which is marked by the inflectional ending -eth) or in the subjunctive (which takes no inflectional ending). Elsewhere in the Book of Mormon text, there are a few cases where an if-clause contains the verb repent and a third person singular subject. In such cases, the indicative -(e)th ending for the verb repent is favored:
In 3 Nephi 18:31, the text originally had repenteth but this indicative form was changed to the subjunctive repent by Joseph Smith in his editing for the 1837 edition. However, the immediately preceding verse retained the original repenteth. Parallelism suggests that this 1837 change in 3 Nephi 18:31 should be reversed and that the original repenteth should be restored.
The example from Mosiah 26:29 (cited above) shows a mixture of the indicative and the subjunctive verb forms, with the subjunctive coming first (“if he confess … and repenteth”). A similar example of mixture in mood is found later on in the text; in this second case, we have two instances of the verb be in an if-clause, the first verb in the subjunctive and the second one in the indicative:
This overall variability implies that we should allow the earliest textual sources to determine whether the verb takes the subjunctive or the indicative in a conditional clause. Here in Mosiah 2:38, Oliver Cowdery’s correction is virtually immediate, so we accept the indicative reading
(“if that man repenteth not”) as the original one. For additional discussion, see 2 Nephi 9:20; also see mood in volume 3.
Summary: Retain in Mosiah 2:38 Oliver Cowdery’s almost immediately corrected reading, the indicative repenteth in the if-clause (“if that man repenteth not”).