11 And if anyone, whose conscience lacks knowledge, should dispute about this and say:— “Since Christ is one and His Father is one, how does Christ dwell, and His Father dwell, in faithful men? And how do righteous men become temples for God that He should dwell in them? If then it is thus, that to each several faithful man there comes a several Christ, and God Who is in Christ—if it is so, there are for them Gods many and Christs without number.” But hear, my beloved, the defence that is suited to this argument.
From that which is visible let him that has thus said receive instruction. For every man knows that the sun is fixed in the heavens, yet its rays are spread out in the earth, and (light) from it enters by many doors and windows of houses; and wherever the sunshine falls, though it be but as (the measure of) the palm of the hand, it is called the sun. And though it fall in many places, it is thus called, but the real sun itself is in heaven. Therefore, if it is so, have they many suns?
Also the water of the sea is vast, and when you take one cup from it, that is called water. And though you should divide it into a thousand vessels, yet it is called water by its name. Also when you kindle fire from fire in many places, the place from whence you take it, when you kindle it, lacks not, and the fire is called by one name. And because you divide it into many places, it does not on that account become possessed of many names. And when you take dust from the earth, and castest it into many places, it is not a whit diminished, and also you can not call it by many names.
Thus also God and His Christ, though they are One, yet dwell in men who are many. And they are in heaven in person, and are diminished in nothing when they dwell in many; as the sun is not a whit diminished in heaven, when its power is poured out in the earth. How much greater then is the power of God, since by the power of God the very sun itself subsists.
Source: Demonstrations (New Advent)