1....We are taught in this Psalm, when we chaunt Allelujah, which means, Praise the Lord, that we should, when we hear the words, “Confess unto the Lord”, praise the Lord. The praise of God could not be expressed in fewer words than these, “For He is good.” I see not what can be more solemn than this brevity, since goodness is so peculiarly the quality of God, that the Son of God Himself when addressed by some one as “Good Master,” by one, namely, who beholding His flesh, and comprehending not the fullness of His divine nature, considered Him as man only, replied, “Why do you call Me good? There is none good but one, that is, God.” And what is this but to say, If you wish to call Me good, recognise Me as God? But since it is addressed, in revelation of things to come, to a people freed from all toil and wandering in pilgrimage, and from all admixture with the wicked, which freedom was given it through the grace of God, who not only does not evil for evil, but even returns good for evil; it is most appropriately added, “Because His mercy endures for ever.”
2. “Let Israel now confess that He is good, and that His mercy endures for ever”. “Let the house of Aaron now confess that His mercy endures for ever”. “Yea, let all now that fear the Lord confess that His mercy endures for ever”. You remember, I suppose, most beloved, what is the house of Israel, what is the house of Aaron, and that both are those that fear the Lord. For they are “the little and the great,” who have already in another Psalm been happily introduced into your hearts: in the number of whom all of us should rejoice that we are joined together, in His grace who is good, and whose mercy endures for ever; since they were listened to who said, “May the Lord increase you more and more, you and your children;” that the host of the Gentiles might be added to the Israelites who believed in Christ, of the number of whom are the Apostles our fathers, for the exaltation of the perfect and the obedience of the little children; that all of us when made one in Christ, made one flock under one Shepherd, and the body of that Head, like one man, may say, “I called upon the Lord in trouble, and the Lord heard me at large”. The narrow straits of our tribulation are limited: but the large way whereby we pass along has no end. “Who shall lay anything to the charge of God's elect?”
3. “The Lord is my helper; I will not fear what man does unto me”. But are men, then, the only enemies that the Church has? What is a man devoted to flesh and blood, save flesh and blood? But the Apostle says, “We wrestle not against flesh and blood, but against,”...he says, “spiritual wickedness in high places;” that is, the devil and his angels; that devil whom elsewhere he calls “the prince of the power of the air.” Hear therefore what follows: “The Lord is my helper: therefore shall I despise mine enemies”. From what class soever my enemies may arise, whether from the number of evil men, or from the number of evil angels; in the Lord's help, unto whom we chant the confession of praise, unto whom we sing Allelujah, they shall be despised.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)