I recently read the phrase "seal you his" and found it intriguing. I searched and found an article about it in the Book of Mormon Central. John Gee notes that the verb to seal occurs 34 times in the Book of Mormon. It usually takes a direct object referring to the law, the word, records, revelation, truth, etc. Twice the direct object is a person by using the expression "seal you his". Quoting from John Gee.
While use of the term to seal to mean “to mark as one’s property, and secure from danger”
was known in Joseph Smith’s day, it was not usually used of persons. What, then, are we to make of the expression “seal you his” in the Book of Mormon? Hebrew seals from before the Babylonian exile (and thus in use during Lehi’s time) provide helpful insight. 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. (Source)
What I find interesting is that the two usages of "seal you his" in the Book of Mormon are polar opposites. One is found in Mosiah 5:15
Therefore, I would that ye should be steadfast and immovable, always abounding in , that Christ, the , may you his, that you may be brought to heaven, that ye may have everlasting salvation and eternal life, through the wisdom, and power, and justice, and mercy of him who all things, in heaven and in earth, who is God above all. Amen.
The other in Alma 34:35
For behold, if ye have procrastinated the day of your repentance even until death, behold, ye have become to the spirit of the devil, and he doth you his; therefore, the Spirit of the Lord hath withdrawn from you, and hath no place in you, and the devil hath all power over you; and this is the final state of the wicked.
In the first case, Christ seals us his. In the other, it is the devil who seals us his. This marks the end of The Two Ways, the destination they lead to, respectively. In one case it brings us to heaven to everlasting salvation and eternal life. In the other, the Spirit has withdrawn completely and the devil has all power. It is "the final state of the wicked". According to the ancient use of seals, we belong to Christ in one case, as his possession, or as king Benjamin states in that same chapter, his sons and daughters, since he has spiritually begotten us (Mosiah 5:7). I find it likely that the covenant King Benjamin's people entered into to become sons and daughters of Christ and sealed to him was marked by some formal ritual.
As mentioned, the more common usage of the word seal in the Book of Mormon is related to the word of God, often in the form of sacred records. A part of the Book of Mormon was sealed. I think there is deeper symbolism to that. We have to be ready for the word of God since it has the power to seal our ultimate fate. It is in the nature of his word
For as I, the Lord God, liveth, even so my cannot return void, for as they go forth out of my mouth they must be fulfilled. (Moses 4:30)
D&C 1 reflects these ideas
8 And verily I say unto you, that they who go forth, bearing these tidings unto the inhabitants of the earth, to them is power given to both on earth and in heaven, the unbelieving and ;
9 Yea, verily, to seal them up unto the when the of God shall be poured out upon the without measure—
10 Unto the when the Lord shall come to unto every man according to his , and to every man according to the measure which he has measured to his fellow man.
38 What I the Lord have spoken, I have spoken, and I excuse not myself; and though the heavens and the earth pass away, my shall not pass away, but shall all be , whether by mine own or by the of my , it is the .