John Calvin Commentary Numbers 3:17

"And these were the sons of Levi by their names: Gershon, and Kohath, and Merari." — Numbers 3:17 (ASV)

And these were the sons of Levi by their names. From this it appears that the tribe of Levi, like the others, had made astonishing progress from a small and humble beginning. For while Levi himself had only fathered three sons—Gershon and Merari only two each, and Kohath four—who would have expected such an increase, that twelve men in so short a time should have grown into so many thousands?

But thus powerfully does God work under the semblance of weakness, so that His glory may be all the more conspicuous. The fact that He promoted the family of Kohath above the others—not only in the priesthood of Aaron but also in their common ministry—proceeded from the same source of His gratuitous good pleasure as did the calling of Moses.

God, then, who had also dignified Moses with such an honorable office, was, for Moses’ sake, also gracious to the family of Kohath. Nevertheless, lest Moses should be suspected of ambition, or lest an occasion for calumny be given to the ungodly, God chose that the sons of Moses should remain in the ordinary station of the Levites.