77 If our foolish opponents are surprised at the difference between the precepts given by God to the ministers of the Old Testament, at a time when the grace of the New was still undisclosed, and those given to the preachers of the New Testament, now that the obscurity of the Old is removed, they will find Christ Himself saying one thing at one time, and another at another. "When I sent you," He says, "without scrip, or purse, or shoes, did ye lack anything? And they said, Nothing.
Then says He to them, But now, he that has a scrip, let him take it, and also a purse; and he that has not a sword, let him sell his garment, and buy one." If the Manichæans found passages in the Old and New Testaments differing in this way, they would proclaim it as a proof that the Testaments are opposed to each other. But here the difference is in the utterances of one and the same person. At one time He says, "I sent you without scrip, or purse, or shoes, and you lacked nothing;" at another, "Now let him that has a scrip take it, and also a purse; and he that has a tunic, let him sell it and buy a sword."
Does not this show how, without any inconsistency, precepts and counsels and permissions may be changed, as different times require different arrangements? If it is said that there was a symbolic meaning in the command to take a scrip and purse, and to buy a sword, why may there not be a symbolic meaning in the fact, that one and the same God commanded the prophets in old times to make war, and forbade the apostles? And we find in the passage that we have quoted from the Gospel, that the words spoken by the Lord were carried into effect by His disciples.
For, besides going at first without scrip or purse, and yet lacking nothing, as from the Lord's question and their answer it is plain they did, now that He speaks of buying a sword, they say, "Lo, here are two swords;" and He replied, "It is enough." Hence we find Peter with a weapon when he cut off the assailant's ear, on which occasion his spontaneous boldness was checked, because, although he had been told to take a sword, he had not been told to use it. Doubtless, it was mysterious that the Lord should require them to carry weapons, and forbid the use of them. But it was His part to give the suitable precepts, and it was their part to obey without reserve.
Source: Reply to Faustus the Manichaean (New Advent)