The Mechaber (151:6) says that some opinions prohibit one from entering the Bais Haknesses with a long knife. The reason one may not enter with a long knife is because the Bais Haknesses is a place that lengthens a person’s life. It is the place where people daven. The knife does the exact opposite- it shortens a person’s life. The Tzitz Eliezer says that when one is davening outside of a Bais Haknesses he should also not be carrying a knife, because davening itself lengthens a person’s life. Since the Bais Haknesses is a place set aside for tefillah a knife is prohibited even not during davening. The Kaf Hachaim says in the name of mekubalim that when davening one should not carry on him anything made from iron.
The poskim say that one may not enter the Bais Haknesses with a gun. The Ohr L’Tziyon and Reb Elyashiv zt”l both say that if he has nowhere else to keep it or it is a time of war he may bring it in. If he is able to conceal it that it can’t be seen, it may be brought in under any circumstance. The Mishna Berura says that this applies to a knife as well. Other poskim say that if he has nowhere to keep it, then it is better for him to daven by himself rather than to bring it into the Bais Haknesses. The Tzitz Eliezer says that if he unloads the ammunition it is permissible to enter since the gun cannot harm anyone.
The Biur Halacha explains that the prohibition to enter the Bais Haknesses with a knife only applies to those who are prohibited from eating there and have no need for the knife in the Bais Haknesses. However for those who are learning in the Bais Haknesses and Bais Hamedrash the entire day and are permitted to eat there, it is also permissible for them to carry a knife for their personal needs. When they are ready to bentch they should cover it over. The Mishna Berura (180:11) says the reason the knife should be covered during bentching is because the knife shortens one’s life, whereas the shulchan, which represents the mizbe’ach, lengthens one’s life.
The Biur Halacha seems to say that the prohibition to enter with a knife applies to a Bais Hamedrash as well. This is based on the Gemara in Sanhedrin (82b) that it is prohibited to bring weapons into the Bais Hamedrash. The Aruch Hashulchan explains that Torah also lengthens the life of a person therefore one should also not enter a Bais Hamedrash with a knife.
By bircas hamazon there is no difference between a short knife or a long knife, the reason of shortening a person’s life applies to both and they must be covered. The Taz asks why is the halacha of bentching different than the halacha of the Bais Haknesses where the reason that it shortens a person’s life applies only to a long knife and not a short knife? Only a long knife may not be brought in.
The Taz answers that by bentching the knife only needs to be covered which is not that very difficult to do. However when entering the Bais Haknesses, it needs to be removed which involves more effort. Also one may have a need for the knife. Therefore they only prohibited a long knife since that involves more effort to remove.1
Prepared by R’ Avrohom Yehoshua Ziskind
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1Sources: Mishnah Berura, Biur Halacha, Dirshu