The practice of Closed Communion has long been a source of contention between confessional Lutheran bodies like The Lutheran Church--Missouri Synod and other Christian churches. However, a proper understanding of our view of the Lord's Supper should at least show the reason for our practice.
To Lutherans, the Lord's Supper is an expression of the Gospel of Jesus Christ, the message that we are saved through faith in His atoning sacrifice, apart from works of the Law. [Eph 2:8-9, Mt 26:28, Mk 14:24, Lk 22:20, Heb 9:14-16] We believe that the Lord's Supper is an expression of that Gospel, a tangible confirmation of the God-man shedding His blood and allowing His body to be broken on the cross to pay the price for our sin. [Mt 26:28, Mk 14:24]
We also believe, according to Jesus' own words, that Jesus' body and blood are truly present in the bread and wine, and that we receive both bread and wine and Jesus' actual body and blood. [Mt 26:26-29, Mk 14:22-25, Lk 22:15-20, 1Cor 10:16, 11:23-26] Also, we understand the Bible's teaching that the Lord's Supper grants forgiveness of sins to all who trust in the promise attached to the Supper. [Mt 26:28]
In addition to the blessings of forgiveness offered through this sacrament, we also understand the Lord's Supper to be an expression of confession, that is, "saying the same thing." When we commune at the Lord's Table, we express and agreement in the teachings of our church body. [1Cor 10:17, Acts 2:42]
Unfortunately, because of sin, there is a Law side to the Lord's Supper. Because some deny the Real Presence of Christ's body and blood in the bread and wine, in Christian love, we withhold the sacrament in consideration of Saint Paul's warning that those who partake of the supper rejecting the presence of Christ in the elements eat and drink judgment on themselves, sinning against Christ. [1Cor 11:28-29; AC XXIV:28-29] We would never want to contribute to the sin of another Christian by presenting him with an opportunity or temptation to sin, offering him condemnation instead of forgiveness. [1Cor 11:28-29]
Also, because of the doctrinal divisions in the church, we are unable to commune with others of a different confession than ours, even if they believe in the Real Presence. [FC SD VII: 44, Ap XXIV: 68], FC SD VII: 59] Since Communion is an expression of unity, communing together while holding a different confession would be a lie, saying by our actions that we are in full agreement when in fact we are not. [1Cor 10:16-17, 11:27ff] Therefore, for the sake of integrity, we do not commune with congregations with whom we do not have doctrinal agreement, and we request the same of those who visit our churches and come from other confessions.