There is a slight difference in the calls to repentance in verses 50 and 51. In verse 50 it is the Spirit calling all to repentance. That is one of the functions of the Spirit, to testify of truth. That truth is that all should come unto Jehovah, the king of heaven and earth. The combination of “heaven and earth” is a common adjective for Jehovah. Alma provides that expression in an expanded form, and it is the only time we see it in association with Jehovah as king. This may have two reasons. The first is the implication that only Jehovah is now their king, since the Nephites are now under the reign of the judges. The other is that there is a triumphal Messiah expected who is to be king of heaven and of earth. Alma is declaring that the atoning Messiah is also the one who will be that triumphal Messiah. Jehovah will perform both Messianic functions. They will come at different times.
Verse 51’s call to repentance comes through Alma at the Spirit’s invitation. It comes as divine support behind Alma’s call to this congregation. The fact that they must repent to inherit the kingdom of heaven emphasizes the realm where the king of heaven dwells, and ties to verse 50.