CFW Walther was the first President of the LC-MS. Here are two quotes from his book, Pastoral Theology.
"Sadly, as we have discovered, there are many preachers who call themselves Lutheran and, after preparing the holy table to administer the Lord’s Supper, invite to the gift of grace anyone who wants to come—without permitting an examination their faith and life. And they even think this is a most Gospel-centered practice."
"Another characteristic of the Holy Supper—and of the Sacraments in general—is that it belongs to the marks, to the banners of the Church. It is a seal of our doctrine and faith (Rom 4:11; see also 1 Cor 10:21, Ex 12:48). The church in which you participate in the Holy Supper is also the church and its doctrine that you confess. There can be no deeper brotherly community than the one that you join and where you partake of the Holy Supper. “For,” as the holy apostle says, “as often as you eat this bread and drink the cup, you proclaim the Lord's death until he comes” (1 Cor 11:26). And, “because there is one bread, we who are many are one body, for we all partake of the one bread” (1 Cor 10:17). Therefore there is a great difference whether you hear a sermon in a different church community on occasion or participate in their celebration of Holy Communion. You can occasionally listen to a sermon—perhaps to get to know the teaching of that denomination—without participating in a heterodox church service. But Holy Communion is an act of confession. If you commune in a different church body, you are actually joining that body, you are acting as a witness to their teaching, and you are declaring their members to be your brothers and sisters in the faith. This being the case, what do we make of the custom of inviting everyone present to the Holy Supper—without distinguishing their different Confessions or examining them?"
Blessings in Christ,
Pastor John Rickert