Sforno on Numbers 8:19/ספורנו על במדבר ח׳:י״ט

1ואתנה את הלוים, since they are by themselves “given” to perform service for Me, they are now at the disposal of Aaron and his sons, My delegates on earth.

2לעבוד את עבודת בני ישראל באהל מועד, to perform the very duties which, previously, it had been the duty of the firstborn to perform on behalf of the Children of Israel.

3ולכפר על בני ישראל, and to be the instrument of the people’s atonement by their accepting the tithes tendered them by the people so that they would be free to perform the duties G’d has now assigned to them. G’d had come to despise the firstborn, seeing that they participated also in the sin of the golden calf which had caused G’d so much grief.

4 ולא יהיה בבני ישראל נגף, both among the Israelites and among the Levites.

5בגשת בני ישראל אל הקודש, the Israelites’ sin would be that of entering domains out of bounds to them, whereas the Levites’ sin would be allowing them to do so without trying to prevent them from entering holy precincts. All of them would make themselves guilty and would die at the hands of G’d as spelled out in 18,3.