From twenty years old and upward, all that are able to go forth to war in Israel: thou and Aaron shall number them by their armies.
God's order to number Israel came with careful limits: males of war age only, family by family, so each man knew his place and the government knew where to find him. When David numbered the people without commission long after, he paid dearly for it. The church is militant, and only those enlisted as soldiers of Jesus Christ are true members.
AI summary
Commenting on Numbers 1:1-16
From twenty years old and upwards,.... All that had entered into their twentieth year, or, as it should rather seem, who were full twenty years of age, and all that were above it without any limitation; though some limit it to fifty, and others to sixty years, when men may be reasonably excused going to war; for to know who were fit for it seems...
Aaron shall number them by their armies--or companies. In their departure from Egypt they were divided into five grand companies (Exo 13:18), but from the sojourn in the wilderness to the passage of the Jordan, they were formed into four great divisions. The latter is here referred to.