1 This Psalm is the first of those to which is prefixed the word Allelujah; the meaning of which word, or rather two words, is, Praise the Lord. For this reason he begins with praises: “O confess unto the Lord, and call upon His Name”; for this confession is to be understood as praise, just as these words of our Lord, “I confess to You, O Father, Lord of heaven and earth.” For after commencing with praise, calling upon God is wont to follow, whereunto he that prays does next add his longings: whence the Lord's Prayer itself has at the commencement a very brief praise, in these words, “Our Father which art in Heaven.” The things prayed for, then follow....This also follows, “Tell the people what things He has done;” or rather, to translate literally from the Greek, as other Latin copies too have it, “Preach the Gospel of His works among the Gentiles.” Unto whom is this addressed, save unto the Evangelists in prophecy?
Source: The Enarrations, or Expositions, on the Psalms (New Advent)