There is nothing better than oil. It is the cause of light, and there also it is the cause of light. “Then shall your light break forth as the morning”, says the Prophet, if you show pity upon your neighbour. And as natural oil contains light, so then does mercy [alms] grant us a great, a marvelous light. Much mention does Paul, too, make of this mercy. In one place, hear him say, “Only that we should remember the poor.” And in another, “If it be meet for me to go also.” And in every place, turn where you will, you see him anxious about this very thing.
And again, “And let our people also learn to maintain good works.” And again, “These things are good and profitable unto men.” Listen to a certain other one who says, “Alms do deliver from death”; If You take away pity, “Lord, Lord, who shall stand”; and it is said, If You enter “into judgment with your servant”; “A great thing is man”; why? “and an honorable thing is a merciful man.” For this is the true character of man, to be merciful, yea rather the character of God, to show mercy.
Do you see, how strong is the mercy of God? This made all things, this formed the world, this made the angels, it was through mere goodness. For this cause, too, He threatened hell, that we may attain unto the kingdom, and through mercy we do attain unto the kingdom. For wherefore did God, being alone, create so many beings? Was it not through goodness? Was it not through love to men? If you ask why such and such things are, you will always find your answer in Goodness. Let us show mercy to our neighbors, that mercy may be shown to us.
These acts of mercy we show not so much to them, as lay up for ourselves against That Day. When the flame of the fire is great, this oil (mercy) is that which quenches the fire, and this brings light to us. Thus by this means shall we be freed from the fire of hell. For whence will He be compassionate and show mercy? Mercy comes of love! Nothing incenses God so much as to be pitiless. “A man was brought to him who owed him ten thousand talents, and he was moved with compassion, and forgave him.
And there were owing to that man from his fellow-servant a hundred pence, and he caught him by the throat. Therefore the Lord delivered him to the tormentors, till he should pay what was due.” Let us on hearing this be merciful to those who are our debtors in money or in sins. Let no one remember evils, if at least he does not wish to injure himself; for he does not so much aggrieve the other (as he injures himself). For he either will follow him with vengeance, or he has not done so; but dost you yourself, while not forgiving your neighbor his sins, seek for a kingdom? Lest this should happen to us, let us forgive all, (for it is ourselves that we pardon,) that God may forgive us our sins, and so we may obtain the good things which are in store, through the grace and lovingkindness, etc.
Source: Homilies on Philippians (New Advent)