10 To take, for example, this requirement on which Faustus ignorantly grounds his charge against the Old Testament, that a man should take his brother's wife to raise up seed for his brother, to be called by his name; what does this prefigure, but that every preacher of the gospel should so labor in the Church as to raise up seed to his deceased brother, that is, Christ, who died for us, and that this seed should bear His name? Moreover, the apostle fulfills this requirement not now in the typical observance, but in the spiritual reality, when he reproves those of whom he says that he had begotten them in Christ Jesus by the gospel, and points out to them their error in wishing to be of Paul.
"Was Paul," he says, "crucified for you? Or were ye baptized in the name of Paul?" As if he should say, I have begotten you for my deceased brother; your name is Christian, not Paulian. Then, too, whoever refuses the ministry of the gospel when chosen by the Church, justly deserves the contempt of the Church. So we see that the spitting in the face is accompanied with a sign of reproach in loosing a shoe from one foot, to exclude the man from the company of those to whom the apostle says, "Let your feet be shod with the preparation of the gospel of peace;" and of whom the prophet thus speaks, "How beautiful are the feet of them who publish peace, who bring good tidings of good!" The man who holds the faith of the gospel so as both to profit himself and to be ready when called to serve the Church, is properly represented as shod on both feet.
But the man who thinks it enough to secure his own safety by believing, and shirks the duty of benefiting others, has the reproach of being unshod, not in type, but in reality.
Source: Reply to Faustus the Manichaean (New Advent)