Torah Portion: Yisro
When you eventually build a stone altar for Me, do not build it out of cut stone. Your sword will have been lifted against it, you will have profaned it (Shemos 20:22).
It is a negative commandment that we not build a stone altar touched by iron, as it is written, do not build it out of cut stone (Shemos 20:22) — i.e. which was chiseled with an iron implement. If the Altar is built out of cut stone, it is disqualified.
Among the roots of this commandment is the aim that we internalize, from the very first day we construct the Altar, that it is an vehicle through which we receive forgiveness, blessing, and, ultimately, peace. Therefore, to keep this in mind, we were commanded to not do any act with any utensil that lends itself to cause destruction, which is iron-implements, since they are used for destruction and are ever ready to spill blood. And since humans are influenced by their actions, and their thoughts always follow their deeds, it is therefore proper to keep our deeds in line with [proper ideals].
This commandment applies when the Holy Temple stands, to both men and woman. One who transgresses this and constructs an altar using a stone that was fashioned by iron, transgresses a negative commandment and receives lashes.