Strong’s says this about the word adoption from Romans 8:15; “the placing as a son, that is, adoption (figuratively Christian sonship in respect to God):—”
Adoption in Biblical times meant that when you adopted someone, by law, you could never disown them; whereas you could legally disown your own flesh and blood, this was not a possibility with adopted children. What a relief this verse and others like it bring to us! Knowing there is nothing we could do now or in the future, our placement is certain with Him! Read today how complicated the Mormon Church has made it for their followers to understand such a simple truth as adoption.
“By the law of adoption those who receive the gospel and obey its laws, no matter what their literal blood lineage may have been, are adopted into the lineage of Abraham. (Abra. 2: 9-11) “The effect of the Holy Ghost upon a Gentile,” the Prophet says, “is to purge out the old blood, and make him actually of the seed of Abraham.” Such a person has “a new creation by the Holy Ghost.” (Teachings, pp. 149-150.)…Indeed, the faithful are adopted to the family of Christ; they become “the children of Christ, his sons, and his daughters”; they are “spiritually begotten,” for their “hearts are changed through faith on his name,” thus being “born of him,” becoming “his sons and his daughters.”
(Mosiah 5:7.)” Bruce R. McConkie, Mormon Doctrine, 23.