Q. 120. Why hath Christ commanded us to address God thus: “Our Father”?
That immediately, in the very beginning of our prayer, He might excite in us a childlike reverence for, and confidence in God, which are the foundation of our prayer: namely, that God is become our Father in Christ, and will much less deny us what we ask of Him in true faith, than our parents will refuse us earthly things.
 Matthew 6:9;  Matthew 7:9–11; Luke 11:11; Isaiah 49:15.
Q. 121. Why is it here added, “Which art in heaven”?
Lest we should form any earthly conceptions of God’s heavenly majesty, and that we may expect from His almighty power all things necessary for soul and body.
 Jeremiah 23:24;  Acts 17:24; Romans 10:12.
Prayer is the soul’s converse with God his Maker. But God is transcendently holy, and infinite in greatness, whereas man is a finite creature of dust that is unholy on account of sin. This being the case, how can man converse with God without being guilty of gross audacity and presumption? The answer is that he cannot, except in the Lord Jesus Christ who is the God-Man, the only mediator between God and men (1 Tim 2:5), who Himself declared: “No man cometh unto the Father, but by me” (Jn 14:6). Prayer, then, must not be regarded only as a duty, but as a great privilege, which was purchased for the children of God by the Lord Jesus Christ.
But the Lord Jesus Christ did not only make our prayers acceptable to God; He makes our prayers sweet to God, and praying sweet to us because we are not only justified in Him, but are adopted into the family of God in Him. Christ is the only begotten Son of God, and we, on account of our union with Him, are reckoned as His adoptive brethren (Heb 2:12); made joint-heirs with Him (Rom 8:17); and enabled by His Spirit to approach God as our “Abba, Father” (Rom 8:15).
This is the basis of the Lord’s teaching us to come to God as “our Father.” Because of what Christ has done, we must not come to God as a severe judge or a tyrannical king. We must come to Him as our heavenly Father who loves us, is pleased with our petitions, and is ready and able to answer them. This is a great encouragement for us to pray. It is also an assurance that when we pray, God’s answer will be perfect and loving. We know by experience that sometimes a child may go to his father for some thing, which would in fact be harmful for him. An indulgent human father may sometimes give in to the child because he myopically considers the present pleasure of the child but fails to consider the long-term or permanent effect of his indulgence. Not so our heavenly Father who knows all things. When we come unto Him, we know He is fully able to fulfil all our requests; and yet He would only accede to our requests if it would work for our good (Rom 8:28) and His own glory.
But as with all things, it is possible for us to abuse our privilege because of sin. Just as it is easy for a child to take for granted his father’s love and begin to speak to him without due respect, so it is easy for a child of God to forget the great distance between God and him so that he comes to prayer without due regards to God’s majesty and greatness. So it is important for us to remind ourselves that our Father is “in heaven.” He is our heavenly Father. We must never conceive of God in our minds in terms of any earthly images, and we must never speak to Him as if it is our right to do so; much less should we ever come to prayer with the attitude that God owes it to us to do what we desire. We must come to Him with the full realisation that He is “our Father, which art in heaven.”
Heidelberg Catechism >