John Calvin Commentary Joshua 3:5

"And Joshua said unto the people, Sanctify yourselves; for tomorrow Jehovah will do wonders among you." — Joshua 3:5 (ASV)

And Joshua said, etc. Some unusual display of divine power in bringing assistance needed to be shown, so that the reluctance arising from hesitation would not cause delay; and yet, so that the Israelites might depend solely on God's counsel, Joshua does not yet clearly explain the specific nature of the miracle, unless, indeed, we interpret what follows shortly after as part of the same context.

In this lies the true test of faith: to rely so much on God's counsel that one does not keep inquiring too anxiously about the method or the outcome. Since the word קדש sometimes means to prepare, and sometimes to sanctify, and either meaning is suitable, I thought it best to leave the choice open.

For faith prepares us to perceive God's work; and in those times, when God revealed Himself more directly to people, they consecrated themselves by a solemn ritual. Thus we see how Moses, when the Law was proclaimed, sanctified the people as God had commanded. The view held by some commentators, that the people were commanded to purify themselves from defilements merely so that nothing would hinder their crossing of the Jordan, seems too narrow.