5 Of these then, my Brethren, yea, of these let us the rather treat. He who will not see what is evil, he who will not hear what is evil, he that turns away his smell from the unlawful fumes, and his taste from the unlawful food of the sacrifices, he who refuses the embrace of another man's wife, breaks his bread to the hungry, brings the stranger into his house, clothes the naked, reconciles the litigious, visits the sick, buries the dead; he surely is a virgin, surely he has lamps.
What seek we more? Something yet I seek. What do you seek yet, one will say? Something yet I seek; the Holy Gospel has set me on the search. It has said that even of these, virgins, and carrying lamps, some are wise and some foolish. By what do we see this? By what make the distinction? By the oil. Some great, some exceedingly great thing does this oil signify. Do you think that it is not charity? This we say as searching out what it is; we hazard no precipitate judgment. I will tell you why charity seems to be signified by the oil.
The Apostle says, “I show unto you a way above the rest.” Though I speak with the tongues of men and of Angels, and have not charity, I have become as sounding brass, or a tinkling cymbal. This, that is “charity,” is “that way above the rest,” which is with good reason signified by the oil. For oil swims above all liquids. Pour in water, and pour in oil upon it, the oil will swim above. Pour in oil, pour in water upon it, the oil will swim above. If you keep the usual order, it will be uppermost; if you change the order, it will be uppermost. “Charity never falls.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)