Lehi lived the law of Moses. “According to the Mosaic law, an altar was to be built either of earth or stones not ‘polluted’ by any tools (Ex. 20:24, 25).” (Reynolds and Sjodahl, Commentary on the Book of Mormon, 1:26).
“Lehi built an altar of stones to make an offering and give thanks. It was an altar of unhewn stones as stipulated in Exodus 20:25. The wording is intentional, again showing the Book of Mormon to be translated from an ancient Semitic record. It was not a ‘stone altar,’ which might allow for cut, fitted stones, but an ‘altar of stones’” (Ogden and Skinner, Book of Mormon, 1:20–21).