Here are a few quotes about prayer from Martin Luther:
We know that our defense lies in prayer alone. We are too weak to resist the devil and his vassals. Let us hold fast to the weapons of the Christian; they enable us to combat the devil. For what has carried off these great victories over the undertakings of our enemies which the devil has used to put us in subjection, if not the prayers of certain pious people who rose up as a rampart to protect us? Our enemies may mock at us. But we shall oppose both them and the devil if we maintain ourselves in prayer and if we persist in it. For we know that when a Christian prays in this way: ‘Dear Father, thy will be done,’ God replies to him, ‘Dear child, yes, it shall be done in spite of the devil and of the whole world.’
Prayer is, therefore, a special exercise of faith, and faith makes the prayer so acceptable that either it will be surely granted, or something better than we ask will be given in its stead.
A Christian is always praying, whether he is sleeping or waking, for his heart is always praying, and even a little sigh is a great and mighty prayer.