When hope is abandoned, the reason is usually to be found in the powerlessness of him from whom help was expected. The confidence characteristic of hope is not wholly grounded on the mere willingness to help professed by him on whom our hope rests: power to help must also be present. We sufficiently express our conviction that the divine will is ready to help us when we proclaim that God is our Father. But to exclude all doubt as to the perfection of His power, we add: “who art in heaven.”
Thomas Aquinas, Compendium of Theology, Part 2, Chapter 6.