18 Be therefore poor, be ye among the members of that widow, let your help be solely in God alone. Money is nought; not thence will you have aid. Many have been cast headlong down for money's sake, many have perished on account of money; many for the sake of their riches have been marked out by plunderers; they would have been safe, had they not had what made men hunt for them. Many have presumed in their more powerful friends: they in whom they presumed have fallen, and have involved in their ruin those who trusted in them.
Look back upon the instances to be seen in the human race. Is it anything singular that I am telling you? We speak these things not only from these Scriptures; read them in the whole world. Take heed that you presume not in money, in a friend, in the honour and the boasting of the world. Take away all these things: but if you have them, thank God if you despise them. But if you are puffed up by them; think not when you will be the prey of men; already are you the Devil's prey. But if you have not trusted in these things, you will be among the members of that widow, who is the Church, of whom it is said, “I will bless her widow with blessings;” you will also be poor, and one of those of whom it is said, “And will satisfy her poor with bread.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)