Quoting from a sermon of Dr. Martin Luther preached in his home on Maundy Thursday in 1534:
Another benign effect of this Sacrament is the union, in faith and doctrine, which it produces among Christians, and which is so very necessary. To bring about true union among Christians it is not sufficient that they come together to hear the same preaching and the same word, but they must also meet around the same altar to receive the same food and drink. One may, perchance, hear me preach the word and yet be my enemy; but if one partakes of the Lord’s Supper he, by that act, makes for himself, individually, a public confession of his faith, although there may be hypocrites now and then; and thus a more reliable union, between the Christians who unite in this Sacrament, is formed than if they merely had the Gospel preached unto them, though this may also cause them to be of one mind. Those of the same faith and the same hope unite at the Table of the Lord, while those of a different faith stand aloof. Agreement in the church is very desirable, and there should be no divisions in matters of faith. This union was properly called by a Latin term, Communio, a communion, and those who would not agree with other Christians in faith, doctrine, and life, were called Excommunicati, as being different in their belief and conduct, and hence unworthy to belong to the congregation of those who are of one mind, lest they might produce dissensions and schisms. By means also of the Holy Sacrament Christ establishes this union among the little company of His believers.
This quote is taken from the inaugural edition of Logia in 1992, in an article titled “Two Sermons on the Holy Supper.” They in turn were quoting from the house postil, in this case from Sermons on the Gospels for the Sundays and Principal Festivals of the Church Year by Martin Luther, translated by E. Schmid and edited by M. Loy (Rock Island, Illinois: Augustana Book Company, 1871).