The Heidelberg Catechism: Lord's Day 46
120. Question: Why has Christ commanded us to address God as Our Father?
Answer: To awaken in us at the very beginning of our prayer that childlike reverence and trust toward God which should be basic to our prayer: God has become our Father through Christ and will much less deny us what we ask of Him in faith than our fathers would refuse us earthly things.
 Matt. 7:9-11; Luke 11:11-13.
121. Question: Why the words "in heaven"?
Answer: These words teach us not to think of God's heavenly majesty as something earthly,  and to expect everything for body and soul from his almighty power.
 Jer. 23:23-24; Acts 17:24-25  Matt. 6:25-34; Rom. 8:31-32