Thomas Aquinas, Compendium of Theology

Since, in addition to faith, hope is also necessary for our salvation, our Savior, who inaugurated and perfected our faith by instituting the heavenly sacraments, thought it well to carry us on to a living hope by giving us a form of prayer that mightily raises up our hope to God. Thus we are taught by God Himself what we ought to request from Him. He would not urge us to pray unless He were determined to hear us; no one asks another for a favor unless he has hope in him, and he asks only what he hopes for. Therefore, in teaching us to ask God for benefits, Christ exhorts us to hope in God, and He shows us what we ought to hope for from Him by making known to us what to request.

Accordingly, we shall go through the petitions contained in the Lord's Prayer, and shall point out all that may relate to the hope of Christians. We shall indicate the person in whom we ought to place our hope, and why, and what we should expect from Him. Our hope ought to be anchored in God to whom we are to pray, as we are told in Psalm 61:9: "Trust in Him," namely, in God, "all ye congregation of people; pour out your hearts before Him," that is, in prayer.