The title "Our Father" also alludes to the greatness of God and what He has done for us. Not just anyone can call God "Father." God has become our father through adoption. The title of God the Father should remind us of the glorious gift of His Son Jesus Christ, for it is only through the blood of Jesus Christ that we might approach God as "Abba, Father" (Gal. 4:6). We are born lost in our sins and dead in our trespasses. We are servants of our fleshly lusts. But through the victory of Jesus Christ on the cross and through His resurrection, we have been redeemed. God gives us faith that unites us to Christ, and we are transferred from the kingdom of darkness into the kingdom of light, from deserving of death and hell to receiving Christ's provision of heaven and life eternal, and from strangers to citizens in the kingdom of God. And God's grace goes even further. God does not leave us mere citizens of his kingdom; He makes us His very children, who eat at His table and become co-heirs of the kingdom with Christ.
This work of Jesus Christ stands as the foundation of the prayer. We can pray to God with trust because of what Christ has done for us. Our prayers should be characterized by trust and a remembrance of what we have been given in faith through Jesus Christ. The catechism calls this the "ground" of prayer. It is the basis, the foundation, the underlying principle, the glue that holds it together, or whatever other metaphor one wishes to use. The point is that the work of Christ is the reason we are able to come before God and call him "Our Father."
This mode of address not only reflects our attitude, but also "awaken(s) in us at the very beginning of the prayer childlike reverence," as the catechism says in Q120. If we were honest, we would admit that we do not always come before the Lord in prayer with the right attitude. Sometimes we come without the trust or the reverence we ought to have. So the beginning of our prayer can serve as a reminder of the proper attitude and reverence. As parents, we train our children to say "Yes, sir" or "Yes, please," instead of merely "Yes." In this way, we teach them to properly answer or ask for things by the words they use. At first when they use the word "sir" or "please" they may not mean it or perhaps even understand it; nevertheless, they are taught how to address others, and that awakens the proper attitude. So it is with the opening of the Lord's Prayer with "Our Father." The disciples asked Jesus how to pray, and He is teaching them. He wants to awaken in them immediately a child-like attitude of trust and reverence toward their heavenly Father. Thus, He begins the prayer, "Our Father who art in heaven," to ensure that the right attitude is awakened within them.
Does this mean that we must start all our prayers with "Our Father?" No. The Lord's Prayer is a model prayer to teach us how to pray. "Our Father" is neither a magical phrase nor a perfect incantation of some sort. It is permissible to start prayers with other phrases, but what is required is that we have the right attitude and mindset when we pray. When we pray we ought to address God as His humble servants who trust Him as a Father. We ought to come to God in faith, knowing the work Christ has done for us.
This also means that the idea of praying to God as if He were our buddy, taught by so many people today, is not quite right. God is indeed our friend, but He is so much more than that. He is also our king, the Lord of lords, and the God of gods. Addressing God as our buddy neglects to incorporate the reverence and trust we must have when we come before the throne of grace in prayer. Prayer does not bring God down to our level. We are not on God's level. In fact, the opening of our prayers exists to direct our thoughts to heaven above. The way we address God must reflect the reality of who He is and His relationship to us. That is why Christ teaches to open the Lord's Prayer: "Our Father."
