"He, therefore, who would receive the Lord’s Supper worthily and for his benefit must previously have come to repentance and faith, must previously have obtained grace and have become a true Christian. Partaking of the Lord’s Supper is in and by itself not beneficial; rather the benefit depends on how one partakes. It does not work ex opere operato. It is not like a medicine which one need merely swallow to have the benefit. It is rather like a treasure house whose treasures can be taken, grasped and held only with the hand of faith."
CFW Walther, Pastoral Theology, as quoted in Francis Pieper, Christian Dogmatics, III: 382, fn 132
The phrase ex opere operato means literally by the very act of the sacrament being performed. It applies to Roman Catholic doctrine wherein a sacrament (not just the Lord's Supper) is effectual simply by being performed provided no mortal sin places an obstacle in the way of beneficial reception of the grace offered. Such mortal sins must be confessed orally and priestly absolution received prior to reception for the sacrament to be received beneficially. For Lutherans the power and deceit of sin is such that we cannot be sure of identifying every mortal sin we have committed, and in any case the only mortal sin that proves an irremovable obstacle to worthy reception of the sacrament of the altar is the lack of faith in our Lord's words of promise: "This is my body given for you...this is my blood of the new testament, shed for you for the forgiveness of sins". Thus the examination of conscience prior to communing has as much or even more to do with faith as with repentance, although faith itself is simply the "open hand" into which God places his treasures of forgiveness, life and salvation.
There is more that can be said, of course, and Walther does so, but this is enough for now.