15 “Behold, themselves shall speak in their mouth, and a sword is on the lips of them”. Here is that sword twice whetted, whereof the Apostle says, “And the sword of the Spirit, which is the Word of God.” Wherefore twice whetted? Wherefore, but because smiting out of both Testaments? With this sword were slain those whereof it was said to Peter, “Slay, and eat.” “And a sword is on the lips of them. For who has heard?” They all speak in their mouth, “Who has heard?”
That is, they shall be angry with men that are slow to believe. They that a little before were even themselves unwilling to believe, do feel disgust from men not believing. And truly, brethren, so it is. You see a man slow before he is made a Christian; you cry to him daily, hardly he is converted: suppose him to be converted, and then he would have all men to be Christians, and wonders that not yet they are. It has chanced out to him at evening to have been converted: but because he has been made hungering like a dog, he has also on his lips a sword; he says, “Who has heard?”
What is, “Who has heard?” “Who has believed our hearing, and to whom has the arm of the Lord been revealed?” “For who has heard?” The Jews believe not: they have turned them to the nations, and have preached. The Jews did not believe; and nevertheless through believing Jews the Gospel went around the city, and they said, “For who has heard?” “And You, Lord, shall deride them”. All nations are to be Christian, and you say, “Who has heard?” What is, “shall deride them”? “As nothing You shall esteem all nations.” Nothing for You it shall be; because a most easy thing it will be for all nations to believe in You.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)