Numbers 8:16

16.For they are wholly given. Lest the other tribes should complain that the number of the people was diminished, God declares that the Levites were alienated from the race of Abraham, since He had acquired them to Himself when He smote all the first-born of Egypt; for it is certain that the first-born of the people, as well as those of their animals, were miraculously rescued from the common destruction. Since, then, God delivered them by special privilege, He thus bound them to Himself by the blessing of their redemption. But this reason would seem no longer to hold good, when God, in demanding the price of redemption, set the first-born free, (178) as was elsewhere stated; else He would require the same thing twice over, which would be unjust. The solution, however, of this is easy; when, in the first census, the first-born of the twelve tribes were counted, they were found to exceed the Levites in number. An exchange was then made, viz., that all the first-born of the twelve tribes, being 22,000 in number, should be free from the tribute, and that God should take the Levites in their place as His ministers. Only 273 were redeemed, because this was the excess of their number above that of the Levites. Thus was it brought to pass, that God was content with these just and equal terms, so as not to oppress the people by a heavy burden. But this compensation, which was only made on that one particular day, did not prevent the Israelites from owing their children, who were not then born, to God. Since, then, this obligation still lay upon them as regarded their posterity, the law was passed that they should redeem their first-born. If any should object that it was not fair for those who should be born of the Levites to be consecrated to God, — I reply, that on this point there was no unfairness, for of whatever tribe they might be descended, they were already His property, together with all their offspring; the condition of the people was not therefore made worse by the exchange; and hence, in all equity, God appointed for the future at what price the Israelites should redeem their first-born. In saying that they were “given” to Him, He means to assert that they were His by compact; (179) and in this sense He declares that from the day in which He smote the first-born of Egypt, the first-born of Israel had become His; and then adds, that He then took the Levites; as much as to say, that He only dealt with his people with respect to the time past.