“How, then, should a man examine or look into himself, so that he might eat and drink worthily in the holy Supper?
“This worthy eating does not consist in a man’s purity, holiness, or perfection. …But by way of contrast with the unworthy, one can understand very easily how that examination or exploration is to be undertaken, namely:
“First, let the mind consider of what nature the act of this Supper is, who is present there, [and] what kind of food is offered and taken there, so that one might prepare himself with due humility and piety for its reception.
“Second, let a man about to approach the Lord’s Table be endowed with the kind of heart that seriously acknowledges his sins and errors, and shudders at the wrath of God, and does not delight in sin, but is troubled and grieved [by it], and has the earnest purpose to amend [his life].
“Third, that the mind sincerely give itself to this concern, that it might not perish in sins under the wrath of God, and therefore with ardent desire thirst for and long for the grace of God, so that by true faith in the obedience, passion, and death of Christ, that is, in the offering of [His] body and shedding of His blood it seek, beg, lay hold and apply to itself the grace of God, forgiveness of sins, and salvation. He that examines and prepares himself in this way, he truly uses this Sacrament worthily, not unto judgment, but unto salvation. …For this medicine has been prepared and provided for the sick who acknowledge their infirmity and seek counsel and help.”
–Martin Chemnitz, Ministry, Word and Sacraments: An Enchiridion, (Concordia Publishing House), 131-2
Lutheran Theology: An Online Journal 