Christians sometimes wonder anxiously, “How can I be sure that God really does hear my prayers? How can I be sure He’ll answer?” Luther astutely takes care of that objection to faithful prayer by reminding us that our prayers are not heard because of our own merits, but because God has commanded us to pray and has promised to hear. A necessary and worthy thought, well worth keeping in mind every day. From the Large Catechism:
But all depends upon this, that we learn also to say Amen, that is, that we do not doubt that our prayer is surely heard, and [what we pray] shall be done. For this is nothing else than the word of undoubting faith, which does not pray at a venture, but knows that God does not lie to him, since He has promised to grant it. 120]Therefore, where there is no such faith, there cannot be true prayer either.
121]It is, therefore, a pernicious delusion of those who pray in such a manner that they dare not from the heart say yea and positively conclude that God hears them, but remain in doubt and say, How should I be so bold as to boast that God hears my prayer? For I am but a poor sinner, etc.
122] The reason for this is, they regard not the promise of God, but their own work and worthiness, whereby they despise God and reproach Him with lying, and therefore they receive nothing. 123] As St. James 1:6 says: But let him ask in faith, nothing wavering; for he that wavereth is like a wave of the sea, driven with the wind and tossed. For let not that man think that he shall receive anything of the Lord. 124] Behold, such importance God attaches to the fact that we are sure we do not pray in vain, and that we do not in any way despise our prayer.
(Large Catechism, Part III, Seventh Petition)