In his farewell address to his people, King Benjamin admonished his people: "Therefore, I would that ye should be steadfast and immovable, always abounding in good works, that Christ, the Lord God Omnipotent may seal you his, that you may be brought to heaven, that ye may have everlasting salvation and eternal life" (Mosiah 5:15)
According to John Gee, Hebrew seals from before the Babylonian exile (and thus in use during Lehi's time) provide helpful insight into King Benjamin's phrase "seal you his." Many of those seals contain a formulaic inscription reading "belonging to," followed by the owner's name. To seal a document or an object, a person would wrap string or twine around it, place a daub of mud on the knot, and press the seal into the mud. Affixing this sort of seal marked the object as the possession of the person in whose name it was sealed.
It is this cultural milieu that underlies the seemingly peculiar usage in the Book of Mormon and clarifies its meaning: our actions allow either Christ or the devil (see Alma 34:35) to place his seal on us to indicate to whom we belong. [John Gee, "Book of Mormon Word Usage: 'Seal You His'," in Insights, Vol. 22, 2002, p. 4]