Numbers 25:7

7.And when Phinehas, the son of Eleazar. The courage of Phinehas is celebrated, who, whilst the rest were hesitating, inflamed with holy zeal, hastens forward to inflict punishment. The backwardness of others is therefore condemned by implication, though their tears were praiseworthy; but, since they were almost stupified by grief, their virtue was not dear from all defect. And certainly, whilst the yet unbridled licentiousness of the people was foaming like a tempestuous sea, we cannot wonder that the minds of the good were altogether or partially disabled. Hence was the zeal of Phinehas the more distinguished, when he did not hesitate to provoke so many worthless and wicked persons infuriated by their lechery. If any object that he transgressed the limits of his calling, when he laid hold of the sword with which God had not armed him, to inflict capital punishment, the reply is obvious, that our calling is not always confined to its ordinary office, inasmuch as God sometimes requires new and unusual acts of His servants. As a priest, it was not the office of Phinehas to punish crime, but he was called by the special inspiration of God, so that, in his private capacity, he had the Holy Spirit as his guide. These circumstances, indeed, ought not to be regarded as an example, so that a general rule may be laid down from them; though, at the same time, God preserves His free right to appoint His servants by privilege to act in His behalf as He shall see fit. God’s judgment of this case may be certainly inferred from its approval, so that we may correctly argue that Phinehas was under His own guidance, since He immediately afterwards declared that He was pleased with the act, as is also stated in Psalms 106:30

Now, if any private person should in his preposterous zeal take upon himself to punish a similar crime, in vain will he boast that he is an imitator of Phinehas, unless he shall be thoroughly assured of the command of God. Let the answer of Christ, therefore, always be borne in mind by us, whereby he restrained His disciples, when they desired, like Elijah, to pray that those who had not received them should be destroyed by fire from heaven, “Ye know not what manner of spirit ye are of.” (Luke 9:54.) In order, therefore, that our zeal may be approved by God, it must be tempered by spiritual prudence, and directed by His authority; in a word, the Holy Spirit must go before and dictate what is right.