And they commanded the people, saying, When ye see the ark of the covenant of the LORD your God, and the priests the Levites bearing it, then ye shall remove from your place, and go after it.

Wesley's Notes

3:3 Commanded the people - In Joshua's name, and by his authority. Priests and Levites - Who were not only Levites, but priests also. For altho' the Levites were to carry the ark, Numb 4:1 - 15, yet the priests might perform that office, and did so upon some solemn occasions. Go after it - Towards Jordan, to go over it in such a manner as I am about to describe. 'Till this time the ark went in the middle of the cloudy pillar, now it goes in the front. Probably the pillar of fire and cloud was still hovering over the ark.

Jamieson-Fausset-Brown Bible Commentary

3, 4. When ye see the ark ., and the priests the Levites bearing it-The usual position of the ark, when at rest, was in the center of the camp; and, during a march, in the middle of the procession. On this occasion it was to occupy the van, and be borne, not by the Kohathite Levites, but the priests, as on all solemn and extraordinary occasions (compare Nu 4:15; Jos 6:6; 1Ki 8:3-6).

then ye shall . go after it. Yet there shall be a space between you and it-These instructions refer exclusively to the advance into the river. The distance which the people were to keep in the rear of the ark was nearly a mile. Had they crowded too near the ark, the view would have been intercepted, and this intervening space, therefore, was ordered, that the chest containing the sacred symbols might be distinctly visible to all parts of the camp, and be recognized as their guide in the untrodden way.

Matthew Henry's Concise Commentary

3:1-6 The Israelites came to Jordan in faith, having been told that they should pass it. In the way of duty, let us proceed as far as we can, and depend on the Lord. Joshua led them. Particular notice is taken of his early rising, as afterwards upon other occasions, which shows how little he sought his own ease. Those who would bring great things to pass, must rise early. Love not sleep, lest thou come to poverty. All in public stations should always attend to the duty of their place. The people were to follow the ark. Thus must we walk after the rule of the word, and the direction of the Spirit, in everything; so shall peace be upon us as upon the Israel of God; but we must follow our ministers only as they follow Christ. All their way through the wilderness was an untrodden path, but most so this through Jordan. While we are here, we must expect and prepare to pass ways that we have not passed before; but in the path of duty we may proceed with boldness and cheerfulness. Whether we are called to suffer poverty, pain, labour, persecution, reproach, or death, we are following the Author and Finisher of our faith; nor can we set our feet in any dangerous or difficult spot, through our whole journey, but faith will there see the prints of the Redeemer's feet, who trod that very path to glory above, and bids us follow him, that where he is, we may be also. They were to sanctify themselves. Would we experience the effects of God's love and power, we must put away sin, and be careful not to grieve the Holy Spirit of God.