14 Forasmuch then as we have partaken of so great a gift, let us do everything not to dishonor such a benefit. For if even before this honor, what was done was worthy of punishment, much more now, after this unspeakable benefit. And this I say not now for no cause, but because I see many after their baptism living more carelessly than the uninitiated, and having nothing peculiar to distinguish them in their way of life. It is, you see, for this cause, that neither in the market nor in the Church is it possible to know quickly who is a believer and who an unbeliever; unless one be present at the time of the mysteries, and see the one sort put out, the others remaining within.
Whereas they ought to be distinguished not by their place, but by their way of life. For as men's outward dignities are naturally to be discovered by the outward signs with which they are invested, so ours ought to be discernible by the soul. That is, the believer ought to be manifest not by the gift only, but also by the new life. The believer ought to be the light and salt of the world. But when thou dost not give light even to yourself, neither bind up your own gangrene, what remains, whereby we are to know you?
Because you have entered the holy waters? Nay, this to you becomes a store of punishment. For greatness of honor is, to them who do not choose to live worthy of the honor, an increase of vengeance. Yea, the believer ought to shine forth not only by what he has received from God, but also by what he himself has contributed; and should be discernible by everything, by his gait, by his look, by his garb, by his voice. And this I have said, not that display, but that the profit of beholders, may be the rule by which we frame ourselves.
Source: Homilies on the Gospel of St. Matthew (New Advent)