75 In like manner, also, when Matthew tells us that the shoes were not to be carried with them on the journey, what is intended is the checking of that care which thinks that such things must be carried with them, because otherwise they might be unprovided. Thus, too, the import of what is said regarding the two coats is, that none of them should think of taking with him another coat in addition to the one in which he was clad, as if he was afraid that he might come to be in want, while all the time the power (which was received from the Lord) made him sure of getting what was needful.
To the same effect, when Mark says that they were to be shod with sandals or soles, he gives us to understand that this matter of the shoe has some sort of mystical significance, the point being that the foot is to be neither covered, nor yet left bare to the ground; by which the idea may be conveyed that the gospel was neither to be concealed, nor yet made to depend on the good things of earth. And as to the fact that what is forbidden is neither the carrying nor the possessing of two coats, but more distinctly the putting of them on—the words being, “and not put on two coats,”— what counsel is conveyed to them therein but this, that they ought to walk not in duplicity, but in simplicity?
Source: The Harmony of the Gospels (New Advent)