18 But again you ask yet another testimony of the time. The Lord said unto Me, You are My Son; this day have I begotten You: and a few words further on, You shall rule them with a rod of iron. I have said before that the kingdom of the Romans is clearly called a rod of iron; but what is wanting concerning this let us further call to mind out of Daniel. For in relating and interpreting to Nebuchadnezzar the image of the statue, he tells also his whole vision concerning it: and that a stone cut out of a mountain without hands, that is, not set up by human contrivance, should overpower the whole world: and he speaks most clearly thus; And in the days of those kingdoms the God of heaven shall set up a kingdom, which shall never be destroyed, and His kingdom shall not be left to another people.
Source: Catechetical Lectures (New Advent)