John Calvin Commentary Deuteronomy 27:5

"And there shalt thou build an altar unto Jehovah thy God, an altar of stones: thou shalt lift up no iron [tool] upon them." — Deuteronomy 27:5 (ASV)

And there shalt thou build an altar. At their first entrance into the land, God commands that a sacrifice of thanksgiving should be offered to Him; and this Joshua performed, as is related in Joshua 8:30-31.

Then Joshua built an altar unto the Lord God of Israel in Mount Ebal;
as Moses the servant of the Lord commanded the children of Israel,
an altar of whole stones, over which no man hath lift up any iron.

First of all, then, this testimony of their gratitude is required, so that the children of Israel, as soon as they have begun to set foot in the land of Canaan, might celebrate the praises of the Lord.

Secondly, he forbids all artificial work because if the altar had been permanent, it would have been an occasion of superstition, and this exceptional instance would have been more regarded than the perpetual Law of God.

Consequently, the nine and a half tribes were so greatly angered against the two tribes of Reuben and Gad, and the half-tribe of Manasseh, on account of the altar which was built on the bank of the Jordan (Joshua 22), to such an extent that they determined to utterly destroy their brothers, until they had cleared themselves by alleging that they had only built it as a memorial of their brotherly union, and not for sacrifice.

Surely, they were good interpreters of the Law who considered it an unpardonable crime that an altar should be left for future generations to draw the people away from the one sanctuary, and thus to destroy the unity of faith.