John Calvin Commentary Numbers 12:13

"And Moses cried unto Jehovah, saying, Heal her, O God, I beseech thee." — Numbers 12:13 (ASV)

And Moses cried to the Lord. The event now proves what was recently stated, that Moses was of a meek and gentle disposition beyond all other men; for he is not only ready immediately to forgive, but also intercedes with God for them. Thus the presumption of Miriam is best rebuked, for the only hope of safety that remains to her is in the dignity of Moses, which recently she could not endure.

From God's reply, it is clear that the punishment she alone received was intended for the instruction of all. The pride and rashness of Miriam were sufficiently disciplined, but God wished it to be a lesson for everyone, that each person should confine himself to his own proper limits.

Meanwhile, let us learn from this passage to pay due honor to God's judgments, so that they may be sufficient for us as the rule of supreme justice. For if earthly parents are granted such power over their children that they may put them to shame at their discretion, how much more reverence is due to our heavenly Father when He brands us with any mark of disgrace?

This was the reason Miriam was excluded for seven days: not only so that she might grieve alone, but also so that her discipline might be beneficial for all.

This is also addressed to us, so that we may learn to blush whenever God is angry with our sins, and thus that shame may produce in us an aversion to sin.

This special example afterwards became a law, as we have already seen (Deuteronomy 24:9);⁴⁷ for when God commands lepers to be separated, He reminds the people of what He had ordained concerning Miriam, lest, if internal impurity is cherished, its infection may spread beyond ourselves.

⁴⁷ Ante, vol. 2, p. 12..