13 “For he that hates his brother is in darkness, and walks in darkness, and knows not whither he goes.” A great thing, my brethren: mark it, we beseech you. “He that hates his brother walks in darkness, and knows not whither he goes, because the darkness has blinded his eyes.” What so blind as these who hate their brethren? For that you may know that they are blind, they have stumbled at a Mountain. I say the same things often, that they may not slip out of your memory.
The Stone which was “cut out of the Mountain without hands,” is it not Christ, who came of the kingdom of the Jews, without the work of man? Has not that Stone broken in pieces all the kingdoms of the earth, that is, all the dominations of idols and demons? Has not that Stone grown, and become a great mountain, and filled the whole earth? Do we point with the finger to this Mountain in like manner as the moon on its third day is pointed out to men? For example, when they wish people to see the new moon, they say, Lo, the moon! lo, where it is!
And if there be some there who are not sharp-sighted, and say, Where? Then the finger is put forth that they may see it. Sometimes when they are ashamed to be thought blind, they say they have seen what they have not seen. Do we in this way point out the Church, my brethren? Is it not open? Is it not manifest? Has it not possessed all nations? Is not that fulfilled which so many years before was promised to Abraham, that in his seed should all nations be blessed? It was promised to one believer, and the world is filled with thousands of believers.
Behold here the mountain filling the whole face of the earth! Behold the city of which it is said, “A city set upon a mountain cannot be hid!” But those stumble at the mountain, and when it is said to them, Go up; “There is no mountain,” say they, and dash their heads against it sooner than seek a habitation there. Esaias was read yesterday; whosoever of you was awake not with his eyes only but with his ear, and not the ear of the body but the ear of the heart, noted this; “In the last days shall the mountain of the house of the Lord be manifest, prepared upon the top of the mountains.” What so manifest as a mountain?
But there are even mountains unknown, because they are situated in one part of the earth. Which of you knows Mount Olympus? Just as the people who dwell there do not know our Giddaba. These mountains are in different parts of the earth. But not so that Mountain, for it has filled the whole face of the earth, and of it is said, “Prepared upon the top of the mountains.” It is a Mountain above the tops of all mountains. “And,” says he, “to it shall be gathered all nations.” Who can fail to be aware of this Mountain?
Who breaks his head by stumbling against it? Who is ignorant of the city set upon a mountain? But marvel not that it is unknown by these who hate the brethren, because they walk in darkness and know not whither they go, because the darkness has blinded their eyes. They do not see the Mountain: I would not have you marvel; they have no eyes. How is it they have no eyes? Because the darkness has blinded them. How do we prove this? Because they hate the brethren, in that, while they are offended at Africans, they separate themselves from the whole earth: in that they do not tolerate for the peace of Christ those whom they defame, and do tolerate for the sake of Donatus those whom they condemn.
Source: Homilies on the First Epistle of John (New Advent)