“Our Father in heaven.” “Our Father in heaven” (Matthew 6:9). I think those two words — phrases, terms — correspond to the second half and first half. I think the word Father in the Sermon on the Mount is intended to carry very tender connotations. There are reasons. I’ll give you one.
For example, Matthew 6:32. He tells them not to be anxious about food and drink and clothing, and then he says why: because “your heavenly Father knows that you need them all.” The implicit point there is: you don’t need to fret about the most nitty-gritty things, because your dad in heaven is totally there. He knows every financial stress, and he’s on your side. He knows. So don’t be anxious.
“God is a king, and he’s a daddy. He’s holy, and he humbles himself.”
On the other hand, look at Matthew 5:34: “Do not take an oath . . . by heaven, for it is the throne of God.” Don’t swear by heaven. You don’t have any control here. That’s the feeling I get when I hear the word “in heaven.” Father: tender, warm, caring, attentive, intimate. In heaven: watch out; don’t treat him lightly. His kingdom really matters. His name really matters. His will really matters. And he’s a Father. He gives you bread, and he gives you forgiveness, and he helps you fight the fight.
So I don’t think that phrase, “Father in heaven,” is an accident. He is majestic and merciful. He dwells “in the high and holy place, and also with him who is of a contrite and lowly spirit” (Isaiah 57:15). He’s a King, and he’s a daddy. He’s holy, and he humbles himself. He is above, and he’s ready to come close anywhere, anytime you call on him. He has plans for the whole universe, and he has a plan — a really perfect, tailor-made plan — for your life and every little piece of it.
Source: By John Piper, Desiring God