Matthew Henry on Psalms 55:9-15

No wickedness so distresses the believer, as that which he witnesses in those who profess to be of the church of God. Let us not be surprised at the corruptions and disorders of the church on earth, but long to see the New Jerusalem. He complains of one that had been very industrious against him. God often destroys the enemies of the church by dividing them. And an interest divided against itself cannot long stand. The true Christian must expect trials from professed friends, from those with whom he has been united; this will be very painful; but by looking unto Jesus we shall be enabled to bear it. Christ was betrayed by a companion, a disciple, an apostle, who resembled Ahithophel in his crimes and doom. Both were speedily overtaken by Divine vengeance. And this prayer is a prophecy of the utter, the everlasting ruin, of all who oppose and rebel against the Messiah.