I love the writing of J. C. Ryle. He says it like it is. As I was studying for my message on Communion, I came across this quote on what it means to come to the Lord's Supper with unconfessed sin. He sees it as a very serious thing.
"Sinners living in open sin, and determined not to give up ought never to come to the Lord's Table. To do so is a positive insult to Christ, and to pour contempt on His Gospel. It is nonsense to profess we desire to remember Christ's death, while we cling to the accursed thing which made it needful for Christ to die. The mere fact that a man is continuing in sin is clear evidence that he does not care for Christ, and feels no gratitude for the offer of redemption . . . To carry unrepentant sin to the Lord's Table, and there receive the bread and wine, knowing in our own hearts that we and wickedness are yet friends, is one of the worst things man can do, and one of the most hardening to the conscience. If a man must have his sins, and can't give them up, let him by all means stay away from the Lord's Supper. There is such a thing as 'eating and drinking in an unworthy manner' and to our own 'judgment.' To no one do these words apply so thoroughly as to an unrepentant sinner."