31 You must also avoid the sin of covetousness, and this not merely by refusing to seize upon what belongs to others, for that is punished by the laws of the state, but also by not keeping your own property, which has now become no longer yours. “If have not been faithful,” the Lord says, “in that which is another man's, who shall give you that which is your own?” “That which is another man's” is a quantity of gold or of silver, while “that which is our own” is the spiritual heritage of which it is elsewhere said: “The ransom of a man's life is his riches.” “No man can serve two masters, for either he will hate the one and love the other; or else he will hold to the one and despise the other. You cannot serve God and Mammon.” Riches, that is; for in the heathen tongue of the Syrians riches are called mammon. The “thorns” which choke our faith are the taking thought for our life. Care for the things which the Gentiles seek after is the root of covetousness.
But you will say: “I am a girl delicately reared, and I cannot labor with my hands. Suppose that I live to old age and then fall sick, who will take pity on me?” Hear Jesus speaking to the apostles: “Take no thought what you shall eat; nor yet for your body what you shall put on. Is not the life more than meat, and the body than raiment? Behold the fowls of the air: for they sow not, neither do they reap nor gather into barns; yet your heavenly Father feeds them.” Should clothing fail you, set the lilies before your eyes. Should hunger seize you, think of the words in which the poor and hungry are blessed. Should pain afflict you, read “Therefore I take pleasure in infirmities,” and “There was given to me a thorn in the flesh, the messenger of Satan to buffet me, lest I should be exalted above measure.” Rejoice in all God's judgments; for does not the psalmist say: “The daughters of Judah rejoiced because of your judgments, O Lord”? Let the words be ever on your lips: “Naked came I out of my mother's womb, and naked shall I return there;” and “We brought nothing into this world, and it is certain we can carry nothing out.”
Source: Letters (New Advent)