What Does "Our Father" Mean?

"Hence, let him who can learn rightly to begin to pray the Lord’s Prayer; and to know what it signifies for me to call God my Father, and for me most truly and fully to regard and consider myself his dear child and the brother of Christ the Lord, who has shared with me everything that he has and placed me in possession of his eternal treasures. Here examine and ask your own heart, whether without doubt and wavering you can thus say from the bottom of your heart: 'Our Father;' whether you are firmly grounded upon and can be assured before God: I consider myself thy dear child, and thee my dear Father, not because I have merited it, or could ever merit it, but because my dear Lord wants to be my Brother, and of his own accord has proclaimed it and invited me to regard him as my Brother, and has said that he would also regard me as such.

Only begin this, I say, and see how you will succeed in the task; and you will soon discover what an unbelieving knave is hidden in your bosom, and that your heart is too dull to believe it. O, I am such a poor sinner, nature exclaims, how dare I exalt myself so highly, seat myself in heaven and boast that Christ is mine, and I am his brother! For this greatness and glory is so exceedingly high, beyond all human sense, heart and thought, that we cannot comprehend it; even as Paul himself also confesses in Philippians 3:12, that he is pressing on to lay hold of it, but has not yet attained it."

-from a sermon delivered by Martin Luther on Easter Sunday, 1538