Faith makes the reception of the 'sacrament' worthy; and this Faith gives us assurance

"In our discussion of the chief thing we have come to the reception of the sacrament, which we have not yet finished. Today we shall see how we must conduct ourselves here, and also who is worthy to receive the sacrament and who belongs there.
It is very necessary here that your hearts and consciences be well instructed and that you make a big distinction between outward reception and inner and spiritual reception. Bodily and outward reception is that in which a man receives with his mouth the body of Christ and his blood, and doubtless any man can receive the sacrament in this way, without faith and love. But this does not make a man a Christian, for if it did, even a mouse would be a Christian, for it, too, can eat the bread and perchance even drink out of the cup.
I would say in the first place that this reception occurs in faith and is inward and will have Christ. There is no external sign by which we Christians may be distinguished from others except this sacrament and baptism, but without faith outward reception is nothing. There must be faith to make the reception worthy and acceptable before God, otherwise it is nothing but sham and a mere external show, which is not Christianity at all. Christianity consists solely in faith, and no outward work must be attached to it.
But faith (which we all must have, if we wish to go to the sacrament worthily) is a firm trust that Christ, the Son of God, stands in our place and has taken all our sins upon his shoulders and that he is the eternal satisfaction for our sin and reconciles us with the God the Father. he who has this faith is the very one who takes his rightful place at this sacrament, and neither devil nor hell nor sin can harm him. Why? because God is his protector and defender. And when I have this faith, then I am certain God is fighting for me; I can defy the devil, death, hell, and sin, and all the harm with which they threaten me. This is the great, inestimable treasure given us in Christ, which no man can describe or grasp in words. Only faith can take hold of the heart, and not every one has such faith [II Thess. 3:2]." - Martin Luther, Eight Sermons at Wittenberg, Pg. 256-257