John Calvin Commentary Numbers 25:7

"And when Phinehas, the son of Eleazar, the son of Aaron the priest, saw it, he rose up from the midst of the congregation, and took a spear in his hand;" — Numbers 25:7 (ASV)

And when Phinehas, the son of Eleazar. The courage of Phinehas is celebrated. While the others were hesitating, he, inflamed with holy zeal, hurried forward to inflict punishment. The reluctance of others is therefore implicitly condemned, though their tears were praiseworthy. However, since they were almost stupefied by grief, their virtue was not entirely free from defect. And certainly, while the still unrestrained licentiousness of the people was raging like a tempestuous sea, it is no wonder that the minds of the good were completely or partially paralyzed.

Phinehas’s zeal was all the more remarkable because he did not hesitate to confront so many worthless and wicked people, enraged by their lust.

If anyone objects that he transgressed the limits of his calling when he seized the sword—with which God had not armed him—to inflict capital punishment, the reply is clear: our calling is not always confined to its ordinary duties, since God sometimes requires new and unusual actions from His servants. As a priest, it was not Phinehas’s role to punish crime. Instead, he was called by the special inspiration of God, so that, acting as a private individual, he had the Holy Spirit as his guide.

Indeed, these circumstances should not be taken as an example from which a general rule can be established. However, God preserves His sovereign right to specially appoint His servants to act on His behalf as He sees fit. God’s judgment on this case can be clearly inferred from His approval. Therefore, we may rightly argue that Phinehas was guided by Him, since God immediately declared His pleasure with the act, as is also stated in Psalm 106:30-31.

Now, if any private individual, in misguided zeal, should take it upon himself to punish a similar crime, he will boast in vain that he is an imitator of Phinehas, unless he is thoroughly assured of God's command. Therefore, let us always remember Christ's answer, by which He restrained His disciples when they, like Elijah, wanted to pray for fire from heaven to destroy those who had not received them: You do not know what manner of spirit you are of. (Luke 9:54–55). Therefore, for our zeal to be approved by God, it must be tempered by spiritual prudence and directed by His authority. In short, the Holy Spirit must lead and dictate what is right.