It is fitting that I say a few words about prayer. The more efficacious it is, if done as it should be, the more skillfully does our adversary usually hinder it. Prayer is gravely hindered by cowardice of spirit and excessive fear. This often happens when a person thinks about his own unworthiness so much that he does not turn his eyes toward divine kindness. But deep calls to deep, a deep light to a deep darkness, a deep mercy to a deep misery. The human heart is deep and inscrutable, but if my iniquity is great, Lord, much greater is your loving kindness! Therefore when my soul is troubled within me, I remember the abundance of your mercy and I am refreshed by it, and when I enter into my own power, I will not to mind your justice only.
Yet just as there is a danger of prayer being too timid, so on the other hand there is no less danger, but rather more, of its being rash. I do not say that I would take away from sinners their confidence in prayer, but I want them to pray as people who commit sin, not as those who act justly. Let them pray with a contrite heart and humble spirit for forgiveness of sins, like the publican who cries, God, be merciful to me a sinner.
A third danger is if his prayer is lukewarm and does not proceed from a lively affection. Timid prayer dos not pierce heaven, because immoderate fear binds the soul so that prayer, far from flying upward, can not even come out. Being lukewarm, it grows weak in its flight and falls because it has no strength. Rash prayer goes up but falls back; it meets resistance, and far from winning grace, it deserves punishment. Prayer that is faithful, humble and fervent will undoubtedly pierce heaven and it will certainly not return fruitless.
Bernard of Clairvaux, Sermon Four for Lent, Sermons for Lent and the Easter Season, CF 52, pp41-42