5 “There shall go a fire before Him, and burn up His enemies on every side”. We remember having read in the Gospel, He shall say, “Depart into everlasting fire, prepared for the devil and his angels.” I do not think it is said of that fire. Why do I not? Because he speaks of some fire, which shall go before Him, before He comes to judgment. For it is said, that the fire goes before Him, and burns up His enemies on every side, that is, throughout the whole world. That fire will burn after His advent: this, on the contrary, will go before Him.
What fire then is this?...Behold, we have understood the fire that goes before Him, that is to be understood of a kind of temporal punishment of the unbelieving and ungodly: let us understand the fire, if possible, of the salvation of the redeemed also; for thus we had proposed. The Lord Himself says: “I have come to send fire on the earth:” “fire” in the same way as a “sword;” as in another passage He says, that He was not come to send peace, but a sword, upon earth. The sword to divide, the fire to burn: but each salutary: for the sword of His own word has in salutary wise separated us from evil habits.
For He brought a sword, and separated every believer either from his father who believed not in Christ, or from his mother in like manner unbelieving: or at least, if we were born of Christian parents, from his ancestors. For no man among us had not either a grandsire, or great grandsire, or some ancestry among the heathen, and in that unbelief which is accursed before God. We are separated from that which we were before; but the sword which separates, but slays not, has cut between us.
In the same way the fire also: “I have come to send fire upon the earth.” Believers in Him were set on fire, they received the flame of love: and for this reason when the Holy Spirit itself had been sent to the Apostles, It thus appeared: “cloven tongues, like as of fire.” Burning with this fire they set out on their march through the world, to burn and set on fire His enemies on every side. What enemies of His? They who forsaking the God who made them, adored the idols they had made....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)