Mosiah 18

Abinadi asked Noah and his priests -

"...who shall be his seed?" (Book of Mormon Mosiah 15:10)

Then answering his own question, he said -

"...all those who have hearkened unto their words, and believed that the Lord would redeem his people, and have looked forward to that day for a remission of their sins, I say unto you, that these are his seed, or they are the heirs of the kingdom of God.
12 For these are they whose sins he has borne; these are they for whom he has died, to redeem them from their transgressions. And now, are they not his seed?" (Book of Mormon Mosiah 15:11 - 12)

Alma, who believed and recorded the words of Abinadi later provides more incite on become His seed asking the believers are you -

"...willing to bear one another's burdens, that they may be light;
9 Yea, and are willing to mourn with those that mourn; yea, and comfort those that stand in need of comfort, and to stand as witnesses of God at all times and in all things, and in all places that ye may be in, even until death, that ye may be redeemed of God, and be numbered with those of the first resurrection, that ye may have eternal life—
10 Now I say unto you, if this be the desire of your hearts, what have you against being baptized in the name of the Lord, as a witness before him that ye have entered into a covenant with him, that ye will serve him and keep his commandments, that he may pour out his Spirit more abundantly upon you?" (Book of Mormon Mosiah 18:8 - 10)

This willingness wasn't all that was required, for after they made this covenant, Alma -

"...commanded them that they should preach nothing save it were repentance and faith on the Lord, who had redeemed his people.
21 And he commanded them that there should be no contention one with another, but that they should look forward with one eye, having one faith and one baptism, having their hearts knit together in unity and in love one towards another." (Book of Mormon Mosiah 18:20 - 21)

It was when there was no contention among themselvers and "their hearts were knit together in unity and in love one towards another" or in other words, they possessed charity, it is then we are told -

"And thus they became the children of God." (Book of Mormon Mosiah 18:22)