From the Eastern Orthodox perspective, what is the rationale behind the Jesus prayer?

An answer was given from the Orthodox perspective to a question about contemplative prayer, regarding their practice of hesychasm. The stated goals were noble, with much Scriptural support. My question is in regard to the Jesus prayer. Humility is of course required of us as we approach the God Who dwells in unapproachable light (1 Tim. 6:16). The Jesus prayer states this: "Lord Jesus Christ, Son of God, have mercy on me, a sinner!" It is repeated over and over in an attempt to be like the publican in humility. I have, in fact, repeated this same prayer in my heart many times. But does it not in actuality represent doubt, not faith? Do Orthodox Christians believe that Christ's death has already exemplified His mercy to us, and that our role is to simply believe and receive that truth in our hearts, and then live accordingly? In other words, do we need to continually ask for mercy, or is it just a reminder of the mercy that we have already been shown?