1 In the psalm you have heard the groaning of the poor, whose members endure tribulations over the whole earth, even unto the end of the world. Make it your chief business, my brethren, to be among and of these members: for all tribulation is to pass away. “Woe to them that rejoice!” “Blessed,” says the Truth, “are they that mourn, for they shall be comforted.” God has become man: what shall man be, for whom God has become man? Let this hope comfort us in every tribulation and temptation of this life. For the enemy does not cease to persecute; and when he does not openly rage, he plots in secret. How does he plot? “And for wrath, they worked deceitfully.” Thence is he called a lion and a dragon. But what is said to Christ? “You shall tread on the lion and the dragon.” Lion, for open rage; dragon, for hidden treachery. The dragon cast Adam out of Paradise; as a lion, the same persecuted the Church, as Peter says: “For your adversary, the devil, goes about as a roaring lion, seeking whom he may devour.” Let it not seem to you as if the devil had lost his ferocity. When he blandly flatters, then is he the more vigilantly to be guarded against. But amid all these treacherous devices and temptations of his, what shall we do but that which we have heard in the psalm: “And I, when they were troublesome to me, clothed me in sackcloth, and humbled my soul in fasting.” There is one that hears prayer, hesitate not to pray; but He that hears abides within. You need not direct your eyes towards some mountain; you need not raise your face to the stars, or to the sun, or to the moon; nor must you suppose that you are heard when you pray beside the sea: rather detest such prayers. Only cleanse the chamber of your heart; wheresoever you are, wherever you pray. He that hears is within, within in the secret place, which the psalmist calls his bosom, when he says, “And my prayer shall be turned in my own bosom.” He that hears you is not beyond you; you have not to travel far, nor to lift yourself up, so as to reach Him as it were with your hands. Rather, if you lift yourself up, you shall fall; if you humble yourself, He will draw near you. Our Lord God is here, the Word of God, the Word made flesh, the Son of the Father, the Son of God, the Son of man; the lofty One to make us, the humble to make us anew, walking among men, bearing the human, concealing the divine.
2. “He went down,” as the evangelist says, “to Capernaum, He, and His mother, and His brethren, and His disciples; and they continued there not many days.” Behold He has a mother, and brethren, and disciples: whence He has a mother, thence brethren. For our Scripture is wont to call them brethren, not only that are sprung from the same man and woman, or from the same mother, or from the same father, though by different mothers; or, in truth, that are of the same degree as cousins by the father's or mother's side: not these alone is our Scripture wont to call brethren. The Scripture must be understood as it speaks. It has its own language; one who does not know this language is perplexed and says, Whence had the Lord brethren? For surely Mary did not give birth a second time? Far from it! With her begins the dignity of virgins. She could be a mother, but a woman known of man she could not be. She is spoken of as mulier [which usually signifies a wife], but only in reference to her sex, not as implying loss of virgin purity: and this follows from the language of Scripture itself. For Eve, too, immediately she was formed from the side of her husband, and as yet not known of her husband, is, as you know, called mulier: “And he made her a woman [mulier].” Then, whence the brethren? The kinsmen of Mary, of whatever degree, are the brethren of the Lord. How do we prove this? From Scripture itself. Lot is called “Abraham's brother;” he was his brother's son. Read, and you will find that Abraham was Lot's uncle on the father's side, and yet they are called brethren. Why, but because they were kinsmen? Laban the Syrian was Jacob's uncle by the mother's side, for he was the brother of Rebecca, Isaac's wife and Jacob's mother. Read the Scripture, and you will find that uncle and sister's son are called brothers. When you have known this rule, you will find that all the blood relations of Mary are the brethren of Christ.
Source: Tractates on the Gospel of John (New Advent)