Rhetoric: Alma also bears testimony that “the words which have been spoken by our fathers are true.” The reference to the fathers encompasses the whole of Nephite tradition, which Alma testifies is “true,” in the sense that it accords with Yahweh’s will. This tradition, centered on the Atoning Messiah, is a prime point of contention between the church and those outside of the church. I read this passage as Alma’s emphasis that the Messiah’s mission is a core belief that must be preserved.
Alma 5:48
48 I say unto you, that I know of myself that whatsoever I shall say unto you, concerning that which is to come, is true; and I say unto you, that I know that Jesus Christ shall come, yea, the Son, the Only Begotten of the Father, full of grace, and mercy, and truth. And behold, it is he that cometh to take away the sins of the world, yea, the sins of every man who steadfastly believeth on his name.
Rhetoric: Alma now emphasizes the positive aspects of his message. The essential message of the Atoning Messiah is his ability to “take away the sins of the world” and, more specifically, “the sins of every man who steadfastly believeth on his name.”
Significantly, Alma speaks “concerning that which is to come,” a reference to the coming of the Atoning Messiah and also a point of contention with non-church-men. In fact, this particular teaching will become even more important in the next few years before the Lord’s advent.
Translation: Both the printer’s manuscript and the 1830 edition have the phrase: “yea the son of the only begotten of the Father.” Unlike the change in 1 Nephi 11:18 (from “mother of God” to “mother of the Son of God”), nothing in Book of Mormon theology appears to underlie the original text. (See “Excursus: The Nephite Understanding of God,” following 1 Nephi 11.) 1 Nephi 11:18 originally contained a concept that was clarified in later LDS thought. This phrase in Alma, however, has nothing in either the Book of Mormon nor in LDS thought that would confirm Jesus as the “son of the only begotten.” Jesus is the Son and is the only begotten. That connection is not only the modern LDS position but is also consonant with Book of Mormon usage in certain situations.
I think the text is simply in error. I suspect that the original was “the son, the only begotten of the Father” just is it has been edited since the 1837 edition. Skousen agrees with this conclusion: “Literally, the earliest text for these two passages [Alma 5:48 and Alma 13:9] says that there is a son of the Son of the Father, which contradicts all other uses in the Book of Mormon of the phrases ‘Only Begotten Son’ and ‘Only Begotten of the Father.’ Otherwise, these phrases always refer to the Son of God, not to a son of the Son of God.”