Numbers 18

Every only child is a firstborn child; but not every firstborn is an only child. A firstborn child is not only one after whom other children are also born but also one before whom no other child is born. "All that opens the womb," says the Lord to Aaron, "of all flesh that are offered to the Lord, of men and beasts, shall belong to you; only the firstborn of men shall be redeemed with a price and the firstborn of beasts that are unclean." The Word of God defined what was meant by a "firstborn." "All," it says, "that opens the womb." Otherwise, if no child is a firstborn child but only one who subsequently has brothers, the firstborn are not due the priests until others are also born, lest perchance a child be an only child and not the firstborn child, in the event that no other child shall be born subsequently.

For all this people had twelve tribes after the number of the twelve sons of Jacob. What we call tribes are as it were distinct houses and congregations of people. This people, I say, had twelve tribes, out of which twelve tribes one tribe was Judah, out of which came the kings. There was another tribe, Levi, out of which came the priests. To the priests serving the temple no land was allotted. So it was necessary that among twelve tribes all the land of promise should be shared. Having separated one tribe of higher dignity, the tribe of Levi, which was of the priests, there would have remained eleven, unless by the adoption of the two sons of Joseph the number twelve were completed.

Indeed, do you wish to know what the difference is between the priests of God and the priests of Pharaoh? Pharaoh grants lands to his priests. The Lord, on the other hand, does not grant his priests a portion in the land but says to them, "I am your portion." You therefore who read these words, observe all the priests of the Lord and notice what difference there is between the priests, lest perhaps they who have a portion in the land and have time for earthly cares and pursuits may appear not so much to be priests of the Lord as priests of Pharaoh. For it is Pharaoh who wishes his priests to have possessions of lands and to work at the cultivation of the soil not of the soul, to give attention to the fields and not to the law. But let us hear what Christ our Lord admonishes his priests: "He who has not renounced all he possesses," he says, "cannot be my disciple."