John Calvin Commentary Joshua 22:26

"Therefore we said, Let us now prepare to build us an altar, not for burnt-offering, nor for sacrifice:" — Joshua 22:26 (ASV)

Therefore we said. The gross impiety of which they had been accused was now well refuted; and yet they seem not to have been in every respect free from blame, because the Law forbids the erection of any kind of statues. It is easy, however, to excuse this by saying that no kind of statues are condemned except those which are intended to represent God.

To erect a heap of stones as a trophy, or in testimony of a miracle, or a memorial of some signal favor of God, the Law has nowhere prohibited (Exodus 20:4; Leviticus 26:1; Deuteronomy 5:8). Otherwise, Joshua and many holy judges and kings after him would have defiled themselves by profane innovation.

But the only thing displeasing to God was to see the minds of men drawn to and fro, so as to worship him in a gross and earthly manner. The children of Reuben, Gad, and Manasseh do all that is required for their exculpation when they declare that they would use the altar only as a bond of brotherly union; and add a sufficient reason: namely, the danger that, after a long time, the ten tribes might exclude the others as strangers because they did not inhabit the same land.

For as the country beyond the Jordan was not at first included in the covenant, a difference of habitation might ultimately prove a cause of dissension. They therefore act with foresight for their posterity, so that they may be able, by means of the altar as a kind of public document, to defend their right, that they may mutually recognize each other, and unite in common in serving one God.