But as truly as I live, all the earth shall be filled with the glory of the LORD.

Wesley's Notes

14:21 With the glory of the Lord - With the report of the glorious and righteous acts of God in punishing this rebellious people.

Jamieson-Fausset-Brown Bible Commentary

21. all the earth shall be filled with the glory of the Lord-This promise, in its full acceptation, remains to be verified by the eventual and universal prevalence of Christianity in the world. But the terms were used restrictively in respect to the occasion, to the report which would spread over all the land of the "terrible things in righteousness" [Ps 65:5] which God would do in the infliction of the doom described, to which that rebellious race was now consigned.

Matthew Henry's Concise Commentary

14:20-35 The Lord granted the prayer of Moses so far as not at once to destroy the congregation. But disbelief of the promise forbids the benefit. Those who despise the pleasant land shall be shut out of it. The promise of God should be fulfilled to their children. They wished to die in the wilderness; God made their sin their ruin, took them at their word, and their carcases fell in the wilderness. They were made to groan under the burden of their own sin, which was too heavy for them to bear. Ye shall know my breach of promise, both the causes of it, that it is procured by your sin, for God never leaves any till they first leave him; and the consequences of it, that will produce your ruin. But your little ones, now under twenty years old, which ye, in your unbelief, said should be a prey, them will I bring in. God will let them know that he can put a difference between the guilty and the innocent, and cut them off without touching their children. Thus God would not utterly take away his loving kindness.