When the altar was made in the Tabernacle and in the First and Second Temples, stones were used which had never been worked by any iron tool. Even a scratch from an iron implement made the stone unfit for use. 

Nachmanides (1194-1270) presents the classic explanation, that iron in general, and swords in particular, represent violence and destruction. In a place as holy as the Temple, they would defile anything they touched. That is why there was nothing made of iron in the Tabernacle, even the pegs that held the structure together.

This biblical rule still has practical implications. Nachmanides’s older contemporary, Rabbi Elazar of Worms (1176-1238), applied this commandment to our behaviour at meals. Our tables, especially on Shabbat, represent the altar. While the original altar was created to extend life through achieving atonement, a sword is made to shorten life. Therefore the two are fundamentally incompatible, and while we do use knives when we eat (just as they used knives in the Temple service), when we recite the Grace after Meals, the knives should be removed or covered. 

Of course, the analogy between the table and the altar is not exact — if we wanted, we could make our dining room tables out of stones crafted with iron, we could even have a table made of nothing but iron. 

Nevertheless, the symbolism of keeping knives away or out of sight when we bensch remains powerful. 

When faith and violence come together, faith is inevitably profaned. “Religious coercion” is almost a contradiction in terms, because when religion is forced, it is not only worthless is it poisonous. For the sake of faith, we should strive to keep the sword away from our altars.