2 Peradventure, you but imperfectly understand what “of one Substance” is. Take we pains that you may understand it; may God assist both me who speak, and you that hear; me, that I may speak such things as are true and fit for you; and you, that before and above all things you may believe; and then that you may understand as best you can. What then is “of One Substance”? Let me make use of similitudes to you, that what is imperfectly understood may be made clear by example.
As, suppose, God is gold. His Son is gold also. If similitudes ought not to be given for heavenly things from things earthly, how is it written, “Now the Rock was Christ”? So then, Whatsoever the Father is, This is the Son also; as I have said, for example, “The Father is gold, the Son is gold.” For he who says, “The Son is not of the Very Substance which the Father is;” what else says he but, “The Father is gold, the Son is silver”? If the Father be gold, and the Son silver; the Only Son has degenerated from the Father.
A man begets a man; of what substance the father is who begets, of the same substance is the Son who is begotten. What is, “of the same substance”? The one is a man, and the other is a man; the one has a soul; so has the other a soul; the one has a body, so has the other a body; what one is, that is the other.
Source: Sermons on Selected Lessons of the New Testament (New Advent)