II. Slaves and serfs (coloni) are not to be ordained.
Men are admitted commonly to the Sacred Order who are not qualified by any dignity of birth or character: even some who have failed to obtain their liberty from their masters are raised to the rank of the priesthood, as if sorry slaves were fit for that honour; and it is believed that a man can be approved of God who has not yet been able to approve himself to his master. And so the cause for complaint is twofold in this matter, because both the sacred ministry is polluted by such poor partners in it, and the rights of masters are infringed so far as unlawful possession is rashly taken of them. From these men, therefore, beloved brethren, let all the priests of your province keep aloof; and not only from them, but from others also, we wish you to keep, who are under the bond of origin or other condition of service: unless perchance the request or consent be intimated of those who claim some authority over them. For he who is to be enrolled on the divine service ought to be exempt from others, that he be not drawn away from the Lord's camp in which his name is entered, by any other bonds of duty.
Source: Letters (New Advent)