John 13:31
Now He spoke this not to them only, but to all who should believe in Him; since even now, there is nothing else that causes the heathen to stumble, except that there is no love. “But,” says some one, “they also urge against us the absence of miracles.” But not in the same way. “But where did the Apostles manifest their love?” Do you see Peter and John inseparable from one another, and going up to the Temple? Do you see Paul disposed in a like way towards them, and do you doubt?
If they had gained the other blessings, much more had they the mother of them all. For this is a thing that springs from a virtuous soul; but where wickedness is, there the plant withers away. For “when,” it says, “iniquity shall abound, the love of many shall wax cold.” And miracles do not so much attract the heathen as the mode of life; and nothing so much causes a right life as love. For those who wrought miracles they often even called deceivers; but they could have no hold upon a pure life.
While then the message of the Gospel was not yet spread abroad, miracles were with good reason marveled at, but now men must get to be admired by their lives. For nothing so raises respect in the heathen as virtue, nothing so offends them as vice. And with good reason. When one of them sees the greedy man, the plunderer, exhorting others to do the contrary, when he sees the man who was commanded to love even his enemies, treating his very kindred like brutes, he will say that the words are folly.
When he sees one trembling at death, how will he receive the accounts of immortality? When he sees us fond of rule, and slaves to the other passions, he will more firmly remain in his own doctrines, forming no high opinion of us. We, we are the cause of their remaining in their error. Their own doctrines they have long condemned, and in like manner they admire ours, but they are hindered by our mode of life. To follow wisdom in talk is easy, many among themselves have done this; but they require the proof by works.
“Then let them look to the ancients of our profession.” But about them they by no means believe; they enquire concerning those now living. For, “show me,” it says, “your faith by your works”; but this is not the case; on the contrary, seeing us tear our neighbors worse than any wild beast, they call us the curse of the world. These things restrain the heathen, and suffer them not to come over to our side. So that we shall be punished for these also; not only for what we do amiss ourselves, but because the name of God is blasphemed.
How long shall we be given up to wealth, and luxury, and the other passions? For the future let us leave them. Hear what the Prophet says of certain foolish ones, “Let us eat and drink, for tomorrow we die.” But in the present case we cannot even say this, so “many” gather round themselves what belongs to all. So chiding them also, the Prophet said, “Will ye dwell alone upon the earth?” Wherefore I fear lest some grievous thing come to pass, and we draw down upon us heavy vengeance from God. And that this may not come to pass, let us be careful of all virtue, that we may obtain the future blessings, through the grace and lovingkindness of our Lord Jesus Christ, by whom and with whom, to the Father and the Holy Ghost, be glory now and forever, and world without end. Amen.
Source: Homilies on the Gospel of John (New Advent)