Question:
Why do we believe in a closed [or guarded] table, instead of an open table for anyone who is a Christian to break bread?

Answer:
There is a portion in Haggai that seems to have some application to why we have a guarded table. The portion is Haggai 2:12, 13 (JND): “If one bear holy flesh in the skirt of his garment, and with his skirt do touch bread, or pottage, or wine, or oil, or any food — shall it become holy? And the priests answered and said, No. And Haggai said, If one that is unclean by a dead body touch any of these, is it become unclean? And the priests answered and said, It shall be unclean.”

This portion in Haggai seems to go along with what is said in 1 Corinthians 5:6, 7, JND translation: “Your boasting is not good. Do ye not know that a little leaven leavens the whole lump? Purge out the old leaven, that ye may be a new lump, according as ye are unleavened. For also our Passover, Christ, has been sacrificed.”
Both of these passages are saying that when there is sin (or leaven), in our midst, it defiles us and causes us to become ‘unclean’ or it `leavens’ us. The sin not only affects the person doing the sin, but it affects EVERYONE who has fellowship with that person.

Therefore, as we are told in 1 Corinthians 5:12, we are to judge those that are “within”—which includes those who wish to come into fellowship. We are to sanctify ourselves—to set ourselves apart from sin. This is why we believe that the Lord has given His people when gathered as an assembly to guard as to who partakes of His—“the Lord’s Supper” (1 Corinthians 11:20).