The author writes for Christians:
Let this be the first and most important point, that all our prayers must be based and rest upon obedience to God, regardless of who we are, whether we are sinners or saints, worthy or unworthy. We must know that God will not have our prayer treated as a joke. But He will be angry and punish all who do not pray, just as surely as He punishes all other disobedience. Furthermore, He will not allow our prayers to be in vain or lost. For if He did not intend to answer your prayer, He would not ask you to pray and add such a severe commandment to it.
In the second place, we should be more encouraged and moved to pray because God has also added a promise and declared that it shall surely be done for us as we pray. He says in Psalm 50:15, “Call upon Me in the day of trouble; I will deliver you.” And Christ says in the Gospel of St. Matthew, “Ask, and it will be given to you; … for everyone who asks receives” (7:7-8). Such promises certainly ought to encourage and kindle our hearts to pray with pleasure and delight. For He testifies with His own Word that our prayer is heartily pleasing to Him. Furthermore, it shall certainly be heard and granted, in order that we may not despise it or think lightly of it and pray based on chance.
You can raise this point with Him and say, “Here I come, dear Father, and pray, not because of my own purpose or because of my own worthiness. But I pray because of Your commandment and promise, which cannot fail or deceive me.” Whoever, therefore, does not believe this promise must note again that he outrages God like a person who thoroughly dishonors Him and accuses Him of falsehood.
Besides this, we should be moved and drawn to prayer. For in addition to this commandment and promise, God expects us and He Himself arranges the words and form of prayer for us. He places them on our lips for how and what we should pray, so that we may see how heartily He pities us in our distress, and we may never doubt that such prayer is pleasing to Him and shall certainly be answered. This <the Lord’s Prayer> is a great advantage indeed over all other prayers that we might compose ourselves. For in our own prayers the conscience would ever be in doubt and say, “I have prayed, but who knows if it pleases Him or whether I have hit upon the right proportions and form?” Therefore, there is no nobler prayer to be found upon the earth than the Lord’s Prayer. We pray it daily, because it has this excellent testimony, that God loves to hear it. We ought not to surrender this for all the riches in the world.
Large Catechism, part 3, Paragraphs 17-23
Quoted from Concordia: The Lutheran Confessions