Closing out the fifth chapter of Mosiah is an exhortation to righteous living followed by the promise of being sealed to/by God (Mos 5:15):
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, through the wisdom, and power, and justice, and mercy of him who created all things, in heaven and in earth, who is God above all. Amen.
And in fact, you can also get sealed by the devil (Alma 34:35).
The idea of a person being sealed by God is pretty straightforward from a theological perspective. It indicates that God has marked him or her as his “property,” that is, the person so designated will be protected and preserved for the eschatological reward. To be sealed by the devil, then, is to become his property and to be destined to the sort of a reward he gives.
Now there is an interesting little blurb right here, in something called Insight, in which the author suggests that the cultural background for this idea is First Temple:
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.
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 to place his seal on us to indicate to whom we belong.
Hm. It’s tough to make a good case about where something as ubiquitous as marking one’s property might have come from. And my OT homies can rattle off numerous examples of deities or kings sealing ominous sounding items such as the Tablets of Destiny. Be all that as it may, the idea that God might seal humans occurs in the NT, and in prominent places, too. Although I’m no authority on 19th century American religion, I’m gonna guess that lots of folks were familiar with their Bible and so with this idea. In Eph 1:11-13 we have individual believers being sealed. I have used the NRSV because it’s easier to follow than the KJV, but you can check your KJV if you like:
In Christ we have also obtained an inheritance, having been destined according to the purpose of him who accomplishes all things according to his counsel and will, so that we, who were the first to set our hope on Christ, might live for the praise of his glory. In him you also, when you had heard the word of truth, the gospel of your salvation, and had believed in him, were marked with the seal of the promised Holy Spirit.
Let no evil talk come out of your mouths, but only what is useful for building up, as there is need, so that your words may give grace to those who hear. And do not grieve the Holy Spirit of God, with which you were marked with a seal for the day of redemption.
And here it is again, in 2 Corinthians (2 Cor 1:21-22):
But it is God who establishes us with you in Christ and has anointed us, by putting his seal on us and giving us his Spirit in our hearts as a first installment.
And for those of you who are into seals as an indication of an elite status, well, we might have that very nuance right here in Revelation (Rev 7:1-8):
After this I saw four angels standing at the four corners of the earth, holding back the four winds of the earth so that no wind could blow on earth or sea or against any tree. I saw another angel ascending from the rising of the sun, having the seal of the living God, and he called with a loud voice to the four angels who had been given power to damage earth and sea, saying, “Do not damage the earth or the sea or the trees, until we have marked the servants of our God with a seal on their foreheads.”
And I heard the number of those who were sealed, one hundred forty-four thousand, sealed out of every tribe of the people of Israel:
From the tribe of Judah twelve thousand sealed, from the tribe of Reuben twelve thousand, from the tribe of Gad twelve thousand, from the tribe of Asher twelve thousand, from the tribe of Naphtali twelve thousand, from the tribe of Manasseh twelve thousand, from the tribe of Simeon twelve thousand, from the tribe of Levi twelve thousand, from the tribe of Issachar twelve thousand, from the tribe of Zebulun twelve thousand, from the tribe of Joseph twelve thousand, from the tribe of Benjamin twelve thousand sealed.
I dunno. That’s 144,000 seals, right there. I think if I wanted to know whether or not folks in early 19th century America might know about an idea in the BoM, or how they might have understood it, I’d start by having a look at the NT.