Josh 3:3

“Finally, see what is said: "Let the people be at a distance from the ark of the covenant," it says, "by two thousand cubits." The priests and the Levites, however, are very near, and near enough so that the ark of the Lord and the divine law are carried on their own shoulders. Blessed are those who deserve to be very close to God. But remember that it is written, "Those who draw near to me, draw near to fire." If you are gold and silver and have drawn near to the fire, you will shine forth more splendid and glowing because of the fire. But if you are conscious of building "wood, hay and stubble" upon the foundation of your faith, and you approach the fire with such building, you will be consumed. Blessed, therefore, are those who are very near, who are so very near that the fire illumines and does not burn them. Nevertheless, even Israel will be saved; but it will be saved from far away, and it makes its journey not by its own power but by the support and foresight of the priests.”

“Levi. Sigonius thinks that the Caathites performed this office on this as on other occasions. But the Vulgate shews that the priests sometimes carried the ark, perhaps because it was uncovered, chap. vi. 6., and 2 Kings xv. 25. At this period the number of priests was but small. Some of the sons of Eleazar and of Ithamar might be old enough to assist their parents: only two would be necessary at a time, though the Rabbins assign four, (which is not improbable. Theodoret) and pretend that the two who went first were obliged to go backwards, in order that their faces might be turned towards the ark, out of respect. The ark now marked the way for the people, as the cloud had disappeared on the death of Moses. (St. Augustine, q. 3.; Masius.) (Calmet) — It had been carried at the head of the army in the desert. (Haydock)”

“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 Num 4:15; Jos 6:6; Kg1 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.”