Weekly Catechism for 2018 (Heidelberg 1563): LORD’S DAY 46
120. Why has Christ commanded us to address God thus: Our Father?
To awaken in us, at the very beginning of our prayer, that filial reverence and trust toward God, which are to be the ground of our prayer; namely, that God has become our Father through Christ, and will much less deny us what we ask of Him in faith, than our parents refuse us earthly things.
121. Why is it added: Who art in heaven?
That we may have no earthly thought of the heavenly majesty of God; and may expect from His almighty power all things necessary for body and soul.