And they said, If you send away the ark of the God of Israel, send it not empty; but by all means return him a trespass offering: then you shall be healed, and it shall be known to you why his hand is not removed from you.
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George Leo Haydock
If The lords were already determined to send back the ark. But the priests knew that some still would not believe that it was the cause of their affliction. To convince all, they try an experiment, which would decide the matter; and in case the ark went back, some suitable presents must accompany it, as a propitiation (Haydock) for the sin which they would not (Menochius) then doubt had been incurred. (Haydock)
Though God stands in need of nothing, all must acknowledge their dependence on him. The pagans always made some present, when they appeared before their idols or monarchs, and God requires the like testimony of submission, Exodus xxiii. 15.