Albert Barnes Commentary Joel 3:10

"Beat your plowshares into swords, and your pruning-hooks into spears: let the weak say, I am strong." — Joel 3:10 (ASV)

Beat your plowshares into swords - Peace had already been promised as a blessing of the gospel. In His days, foretold Solomon, shall the righteous flourish, and abundance of peace, so long as the moon endureth (Psalms 72:7). And another: He maketh thy borders peace (Psalms 147:14).

Peace within with God flows forth in peace with man. Righteousness and peace kissed each other (Psalms 85:10). Where there is not rest in God, all is unrest.

And so, all that was necessary for life, the means of subsistence, care of health, were to be forgotten for war.

Let the weak say, I am strong - It is one last gathering of the powers of the world against their Maker; the closing scene of man’s rebellion against God. It is their one universal gathering.

None, however seemingly unfit, was to be spared from this conflict; no one was to remain behind. The farmer was to forge for war the instruments of his peaceful toil; the sick was to forget his weakness and to put on a strength he did not have, and that to the uttermost.

But as weakness is, in and through God, strength, so all strength out of God is weakness. Man may say, “I am strong”; but, against God, he remains weak, as it is said, that weak man from the earth may no more oppress (Psalms 10:18).