...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.
From “On Rogationtide Prayer and Procession” by Martin Luther (Vol. 42, pp. 88-89, Luther's Works, American Edition, Augsburg Fortress Press, 1969).