John Calvin Commentary Numbers 35:10

"Speak unto the children of Israel, and say unto them, When ye pass over the Jordan into the land of Canaan," — Numbers 35:10 (ASV)

Speak unto the children of Israel. God appointed the cities of refuge not only to make a distinction between sins of malice and error, but also so that innocent blood would not be rashly shed. Until now, we have seen how severely He intended murder to be punished. However, since it would have been by no means just that someone who had not willfully but accidentally killed his neighbor should be subjected to the same punishment as willful murderers, an exception is added here, so that one who had killed another ignorantly and unintentionally might escape.

Although, as has been said, God had a further object: namely, to prevent murder upon murder from being committed, and to prevent the land from thus being polluted. Let us now examine the details in order. Although at the outset He only mentions the cities on the other side of the Jordan, we still gather from what follows that six cities were chosen for this purpose, three of which were on this side of the Jordan.

He intended for them to be situated so that every part of the country would have one of them in its vicinity, to prevent the exile of the guiltless, unhappy persons from being made more painful by the distance they would have to travel. We have already briefly pointed out⁵² that these cities were to be in the portions of Levi, so that the dignity of the priesthood could better protect the exiles. This was also because it was probable that there would be more prudence and seriousness among the Levites, ensuring that the refuge granted to the innocent would not also shield the guilty.

⁵² See vol. 2 p. 251, on , on Numbers 35:6..