Moroni 6:4 uses the same basic expression as found in the King James Bible: “looking unto Jesus / the author and finisher of our faith” (Hebrews 12:2). We note here that the definite article the is repeated in the Book of Mormon version (“the author and the finisher”) but not in the King James version (“the author and finisher”), nor is the definite article repeated in the original Greek. However, paraphrastic aspects are involved here since Moroni’s quote is in the third person while the King James version is in the first person (the Greek, however, does not have the our). The use of the repeated the is characteristic of the Book of Mormon text, so it is not surprising that there is this difference in comparison to the King James text. The critical text will maintain the repeated the in the phrase “the author and the finisher of their faith”.
Summary: Accept the repeated the in Moroni 6:4 even though Hebrews 12:2 does not have it; here we follow the reading of the earliest textual source, the printer’s manuscript.