We are summoned to this Table in faith. Without faith it is impossible to please God, and we want our time here to be entirely pleasing to Him, as well as to ourselves. This means that we must come with full, complete, entire, evangelical faith in Jesus.
We may come with our sins, because that means we are coming to lay them down. But we may not come in our sins. If we come in our sins, which is to say, if we come in our unbelief, then this has the effect of turning this bread into a crust and the wine into vinegar. This is why Paul could say of some Corinthians that their approach to the Supper was doing more harm that good.
Note that the contrast is not between imperfect sinners coming and perfect saints coming. No, the contrast is between imperfect sinners coming in humility and imperfect sinners coming in insolence and pride. The issue is not the fact of sin—we all confess sin at the beginning of our service for a reason. Any man who says he does not have sin deceives himself.
But any man who says that he must continue to hold onto that sin also deceives himself. Christ came to cleanse sin, forgive it, eradicate it . . . He did not come to move it around in our lives. He did not take up residence in your life and heart in order to rearrange the furniture there. He came to remake all. Our presence here should say that this is exactly what we want Him to be doing. So come, and welcome, to Jesus Christ.