What the Old Testament especially teaches us is this, that zeal is as essentially a duty of all God's rational creatures as prayer and praise, faith and submission; and surely, if so, especially of sinners whom He has redeemed. That zeal consists in a strict attention to His commands, an intense thirst for the advancement of His glory, a carelessness of obloquy or reproach or persecution, a forgetfulness of friend and relative, nay, hatred (so to say) of all that is naturally dear to us, when He says, "Follow me." A certain fire of zeal, showing itself not by force and blood, but as really and certainly as if it did, is a duty of Christians in the midst of all that excellent overflowing charity which is the highest Gospel grace, and the fulfilling of the second table of the Law.
I. Of course it is absolutely sinful to have any private enemies. When David speaks of hating God's enemies, it was under circumstances when keeping friends with them would have been a desertion of the truth. We hate sinners by putting them out of our sight as if they were not, by annihilating them in our affections. But in no case are we to allow ourselves resentment or malice.
II. It is quite compatible with the most earnest zeal to offer kind offices to God's enemies when in distress. God "maketh His sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust."
III. The Christian keeps aloof from sinners in order to do them good. He does so in the truest and most enlarged charity.
A true friend is he who speaks out, and when a man sins, shows him that he is displeased at the sin. The Psalmist speaks in this spirit when after praying God to persecute the ungodly with His tempest, he adds "fill their faces with shame, that they may seek Thy name, O Lord."
J. H. Newman, Parochial and Plain Sermons,vol. iii., p. 173.
Reference: Judges 5:31. J. Van Oosterzee, Year of Salvation,vol. ii., p. 411.