Now therefore make a new cart, and take two milch kine, on which there hath come no yoke, and tie the kine to the cart, and bring their calves home from them: Make a new cart - It was indecent and improper to employ in any part of the worship of God any thing that had before served for a common purpose. Every thing in the worship of God is said to be sanctified: now the general meaning of that word is, to separate a thing from all earthly and common uses, and devote it solely to the service of God.
When David removed the ark from the house of Abinadab, he put it on a new cart, 2 Samuel 6:3.
Bring their calves home from them - So it appears that their calves had been with them in the fields. This was a complete trial: unless they were supernaturally influenced, they would not leave their calves; unless supernaturally directed, they would not leave their home, and take a way unguided, which they had never gone before.