18 and gathered the whole congregation together on the first day of the second month. The people registered themselves in their tribes according to their ancestral families, putting down the names of those who were twenty years old and older,

Matthew Henry's Commentary on Numbers 1:18

Commentary on Numbers 1:1-43

(Read Numbers 1:1-43)

The people were numbered to show God's faithfulness in thus increasing the seed of Jacob, that they might be the better trained for the wars and conquest of Canaan, and to ascertain their families in order to the division of the land. It is said of each tribe, that those were numbered who were able to go forth to war; they had wars before them, though now they met with no opposition. Let the believer be prepared to withstand the enemies of his soul, though all may appear to be peace.