Because repentance includes the covenant to obey the commandments of the Lord, it is tied closely to baptism in water as the public evidence or witness of that covenant. Thus baptism in water is the appropriate sequel to repentance. This is the sense of the puzzling phrase, introduced to Alma by the Lord in answer to his prayer about dealing with transgressors in the church and repeated on numerous later occasions by both Alma and Mormon, that people should be "baptized unto repentance" (Mosiah 26:22). This phrasing can be confusing when we expect repentance to precede baptism, and the preposition unto seems to indicate that baptism precedes repentance. But the Oxford English Dictionary. which gives the most complete historical analysis of the varieties of English usage, lists 29 distinguishable uses for this preposition. The one which corresponds with the Lord's usage here would indicate that baptisms into the church should only occur in accordance, agreement, or correspondence with the prior repentance of the new member. Because the covenant witnessed in baptism is part of repentance. This relationship is signaled exactly by the phrase baptized unto repentance. And so, Alma asks his new converts, "what have you against being baptized in the name of the Lord, as a witness before him that ye have entered into a covenant with him, that ye will serve him and keep his commandments?" (Mosiah 18:10). On the other side of this same story, King Limhi and his people repented and "entered into a covenant with God ... to serve him and keep his commandments" (Mosiah 21 :32). Furthermore, "they were desirous to be baptized as a witness and a testimony that they were willing to serve God with all their hearts,"
Noel Reynolds, "The True Points of My Doctrine", Journal of Book of Mormon Studies, 5/2, 1996
Interesting to me that the word "unto" in the temple is translated, in the Spanish version, to "para con", which is a formal, little used phrase which means "that corresponds to, in connection with, related specifically to, in relationship to".
I think the Spanish language translation got it right.
I think we English speakers, when we encounter the also-puzzling phrase in the temple with "unto" in it, generally leap to one of the other 28 definitions, which are less accurate, and some of which can cause serious dismay if they are assumed to be correct.
Perhaps someday someone will notice that and make a change to that awkward phrase to make the meaning in English more clear. I hope so.