The first part of verse 29 was important to the conclusion of the previous verses, but the ending part of that verse is the introduction to verses 30-32. Again, Mormon pivots from one topic to the next, within the same sentence. Verse 29 spoke of the ministering of angels. Thus, it is angels who “are subject to him (Christ)”, as stated at the beginning of verse 30. There are still miracles, still angels, and the heavens still care for humankind.
This pivots to the important subject of repentance, for “the office of [the angels’] ministry is to call men unto repentance.” Note that repentance prepares humankind for faith in Christ (verse 32). The next two verses continue to explain this interface between repentance and faith.