Jesus encourages us to say, “Our Father.”

Why did Christ command us to address God thus: Our Father?

To awaken in us at the very beginning of our prayer that childlike reverence for and trust in 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.

Matt. 7:9-11
Or which one of you, if his son asks him for bread, will give him a stone? Or if he asks for a fish, will give him a serpent? If you then, who are evil, know how to give good gifts to your children, how much more will your Father who is in heaven give good things to those who ask him!

Luke 11:11-13
What father among you, if his son asks for a fish, will instead of a fish give him a serpent; or if he asks for an egg, will give him a scorpion? If you then, who are evil, know how to give good gifts to your children, how much more will the heavenly Father give the Holy Spirit to those who ask him!”

I Peter 1:17
And if you call on him as Father who judges impartially according to each one’s deeds, conduct yourselves with fear throughout the time of your exile

Isa. 63:16
For you are our Father,
though Abraham does not know us,
and Israel does not acknowledge us;
you, O Lord, are our Father,
our Redeemer from of old is your name.

Why is it added: Who art in heaven?

That we might have no earthly thought of the heavenly majesty of God,

Jer. 23:23-24
Am I a God at hand, declares the Lord, and not a God far away? Can a man hide himself in secret places so that I cannot see him? declares the Lord. Do I not fill heaven and earth? declares the Lord.

Acts 17:24-25, 27
The God who made the world and everything in it, being Lord of heaven and earth, does not live in temples made by man, nor is he served by human hands, as though he needed anything, since he himself gives to all mankind life and breath and everything…that they should seek God, in the hope that they might feel their way toward him and find him. Yet he is actually not far from each one of us.

and from His almighty power expect all things necessary for body and soul.

Rom. 10:12
For there is no distinction between Jew and Greek; for the same Lord is Lord of all, bestowing his riches on all who call on him.

I Kings 8:28
Yet have regard to the prayer of your servant and to his plea, O Lord my God, listening to the cry and to the prayer that your servant prays before you this day.

Psa. 115:3
Our God is in the heavens;
he does all that he pleases.

From The Heidelberg Catechism