When visitors come to visit the church should they participate in the Lord's Supper?
The partaking of the Lord's Supper is not limited to members of the local congregation. Paul, while journeying through Troas on his way to Jerusalem, worshipped with the brethren in Troas. "Now on the first day of the week, when the disciples came together to break bread, Paul, ready to depart the next day, spoke to them and continued his message until midnight" (Acts 20:7). Even though Paul and his traveling companions were not local members, they still were able to worship with fellow brethren.
Generally, though, this question is asked of non-Christians visiting our congregations. Here the key passage is I Corinthians 11:23-31: "For I received from the Lord that which I also delivered to you: that the Lord Jesus on the same night in which He was betrayed took bread; and when He had given thanks, He broke it and said, "Take, eat; this is My body which is broken for you; do this in remembrance of Me." In the same manner He also took the cup after supper, saying, "This cup is the new covenant in My blood. This do, as often as you drink it, in remembrance of Me." For as often as you eat this bread and drink this cup, you proclaim the Lord's death till He comes. Therefore whoever eats this bread or drinks this cup of the Lord in an unworthy manner will be guilty of the body and blood of the Lord. But let a man examine himself, and so let him eat of the bread and drink of the cup. For he who eats and drinks in an unworthy manner eats and drinks judgment to himself, not discerning the Lord's body. For this reason many are weak and sick among you, and many sleep. For if we would judge ourselves, we would not be judged." Notice that while the partaking of the Lord's Supper is done as a congregation, the manner of partaking is individual. It is the individual who is to examine himself and judge his motives rightly. If a person can properly judge himself, then he will not face condemnation at the final judgment.
The judgment is done by the individual because only he can examine his own heart. "For what man knows the things of a man except the spirit of the man which is in him?" (I Corinthians 2:11). It would be an impossible task for those serving the Lord's Supper to grill each participant and have any reasonable hope of making accurate judgments.
Let us assume that one comes visiting who has not yet known Christ or one attends who is openly living contrary to Christ's law. If such a person participates and brings judgment upon himself as Paul warns, is he any worse off than he is already? They already face the danger of God's wrath; they are already in sin. Those serving are not causing him to sin. Partaking in an unworthy manner is a sin one brings upon himself.
For these same reasons, no one should be forced to participate in the partaking of the Lord's Supper. While it is a requirement for eternal life, it must be done correctly, and only the individual can determine if he is doing so for the proper reason. "Then Jesus said to them, "Most assuredly, I say to you, unless you eat the flesh of the Son of Man and drink His blood, you have no life in you. Whoever eats My flesh and drinks My blood has eternal life, and I will raise him up at the last day"" (John 6:53-54).