2 With these preliminary remarks, I think that we shall not have to labor much for the meaning in this chapter; for that is often betokened in the head which was to be in the body. “After these things,” says he, “Jesus walked in Galilee: for He would not walk in Judea, because the Jews sought to kill Him.” This is what I have said; He offered an example to our infirmity. He had not lost power, but He was comforting our weakness. For it would happen, as I have said, that some believer in Him would retreat into concealment, lest he should be found by the persecutors; and lest the concealment should be objected to him as a crime, that occurred first in the head, which should afterwards be confirmed in the member. For it is said, “He would not walk in Judea, because the Jews sought to kill Him,” just as if Christ were not able both to walk among the Jews, and not be killed by them. For He manifested this power when He willed; for when they would lay hold of Him, as He was now about to suffer, “He said to them, Whom do you seek? They answered, Jesus. Then, said He, I am He,” not concealing, but manifesting Himself. That manifestation, however, they did not withstand, but “going backwards, they fell to the ground.” And yet, because He had come to suffer, they rose up, laid hold of Him, led Him away to the judge, and slew Him. But what was it they did? That which a certain scripture says: “The earth was delivered into the hands of the ungodly.” The flesh was given into the power of the Jews; and this that thereby the bag, as it were, might be rent asunder, whence our purchase-price might run out.
3. “Now the Jews' feast of tabernacles was at hand.” What the feast of tabernacles is, they who read the Scriptures know. They used on the holy day to make tabernacles, in likeness of the tabernacles in which they dwelt while they sojourned in the wilderness, after being led out of Egypt. This was a holy day, a great solemnity. The Jews were celebrating this, as being mindful of the Lord's benefits— they who were about to kill the Lord. On this holy day, then (for there were several holy days; but it was called a holy day with the Jews, though it was not one day, but several), “His brethren” spoke to the Lord Christ. Understand the phrase, “His brethren,” as you know it must be taken, for it is not a new thing you hear. The blood relations of the Virgin Mary used to be called the Lord's brethren. For it was of the usage of Scripture to call blood relations and all other near kindred by the term brethren, which is foreign to our usage, and not within our manner of speech. For who would call an uncle or a sister's son “brother”? Yet the Scripture calls relatives of this kind “brothers.” For Abraham and Lot are called brothers, while Abraham was Lot's uncle. Laban and Jacob are called brothers, while Laban was Jacob's uncle. When, therefore, you hear of the Lord's brethren, consider them the blood relations of Mary, who did not a second time bear children. For, as in the sepulchre, where the Lord's body was laid, neither before nor after did any dead lie; so, likewise, Mary's womb, neither before nor after conceived anything mortal.
Source: Tractates on the Gospel of John (New Advent)