1 Chronicles 15:2, commentary and pictures

None but the Levites were to carry the ark of God, for the Lord had chosen them for that carrying and for ministering to him. Within that appointment the sons of Kohath were specially assigned to bear the ark and the sacred vessels on their shoulders, and only after the priests had covered them, without touching them. The ark was not to be transported on a cart by another method, but to be handled according to the service God had assigned and carried according to his directions as a lasting and continual duty.

David stated this rule in his effort to bring the ark from Kirjath-jearim to Jerusalem and to place it in the tent he had prepared. His earlier attempt using a cart ended with the death of Uzzah, while the later bringing of the ark according to the prescribed order was done with gladness and with greater reverence and joy because its holiness had been honored and God's requirements followed. David's use of a cart disregarded God's command and brought judgment upon Uzzah, showing that good intentions do not excuse disobedience and that explicit obedience is required without altering God's commands at will. God's instructions could have been learned from Scripture and from the priests and Levites, and were not to be replaced by human, heathen, or Philistine custom, by the Philistine method, or by attempts to improve them by human invention and human arrangements.

When David recognized the required order after his earlier disobedience and the Levites carried the ark in the appointed manner, God helped the Levites and the ark was brought safely to Jerusalem. The charge to minister "for ever" and continually describes a lasting duty limited to the duration of mortal service, illustrating that the word "forever" can mean only as long as the thing endures and not necessarily for eternity.