But, of course, our private property system -- like our system of production -- is a relatively recent development. And it certainly wasn't taken for granted throughout most of the last century. If we (ethicists and political philosophers now) purport to give more-or-less universal standards of justice, it seems weird to think that we have to take the economic system of one particular historical epoch as fixed.
Here's a nice quotation from Aquinas, via Peter Singer:
Now, according to the natural order instituted by divine providence, material goods are provided for the satisfaction of human needs. Therefore the division and appropriation of property, which proceeds from human law, must not hinder the satisfaction of man's necessity from such goods. Equally, whatever a man has in superabundance is owed, of natural right, to the poor for their sustenance. So Ambrosius says, and it is also to be found in the Decretum Gratiani: ``The bread which you withhold belongs to the hungry; the clothing you shut away, to the naked; and the money you buy in the earth is the redemption and freedom of the penniless.
That's ST II-II, q 68, art 7, trans by Dawson.