...our prayer must not be based upon or depend upon our worthiness or that of our prayer, but on the unwavering truth of the divine promise. Whenever our prayer is founded on itself or something else, it is false and deceptive, even though it wrings your heart with its intense devotion or weeps sheer drops of blood.
We pray after all because we are unworthy to pray. The very fact that we are unworthy and that we dare to pray confidently, trusting only in the faithfulness of God, makes us worthy to pray and to have our prayer answered. Be as unworthy as you may, but know most seriously that it is a thousand times more important, yes, that everything depends on your honoring God's truthfulness and your never giving his promise the lie by your doubts. Your worthiness does not help you; and your unworthiness does not hinder you. Mistrust condemns you, but confidence makes you worthy and upholds you.
All your life you must, therefore, guard against deeming yourself worthy or fit to pray or to receive, unless it be that you proceed with bold courage, trusting in the truthful and certain promises of your gracious God, who thereby wants to reveal his mercy to you. Thus, just as he, unasked, promised fulfillment out of sheer grace to you, an unworthy and undeserving person, he will, in sheer mercy, also give heed to you, an unworthy petitioner. And for all this you have not your own worthiness to thank, but his truth, whereby he has fulfilled his promise, and his mercy, which prompted the promise. This is supported in the statement found in Psalm 25[:10], "All the works of God are mercy and truth," mercy as manifested in the promise, truth in the keeping and fulfillment of the promise. We also find it in the words of Psalm 85[:10], “Mercy and truth have kissed,” that is, they are joined in every work and gift for which we pray, etc.
From “On Rogationtide Prayer and Procession” by Martin Luther (Vol. 42, pp. 88-89, Luther's Works, American Edition, Augsburg Fortress Press, 1969).