“A canticle of David, when the house was built after the captivity.” So far as the literal sense is concerned, the heading points to the time when the temple at Jerusalem is known to have been refurbished by Zerubbabel, son of Salathiel, after it had been leveled to the ground by a hostile band of Chaldeans. But since he says nothing of this kind in what follows, and since the headings of psalms are never at variance with their content, it remains for us to investigate it in the spiritual sense. A destroyed house is built up when a soul following the captivity of sin begins to return to an understanding of the truth through the generosity of the Lord. This house, which is the universal church in which Christ dwells, is always raised up on living stones, because every day it gains increase in building from its confessors and does not cease to be built up until the number of the predestined is attained at the end of the world. We must store this psalm in our minds as the second of those proclaiming the first and the second coming of the Lord.
The whole multitude as if one, etc. Note the grace of the early Church, in which the heart and soul of the multitude of believers was one (Acts IV), being found even in this group of exiles: so that even though there was such a large army, which almost completed the total of fifty thousand, and this army of different rank and condition, nonetheless the whole multitude, on account of the same faith and love, seemed as if it were one man, by the gift of Him who makes those of one mind live in a house. But the servants and handmaids returning from Babylon to Jerusalem hold the type of those in the Church who indeed strive, through the improvement of a better life, to overcome vices and ascend to the pinnacle of virtues; yet they are not sufficient by themselves to provide the way of a regular life, but rather need to be still restrained by the industry of those who have preceded them in Christ, and to be directed to the path of the desired truth. It continues:
Forty-two thousand: Those who are reckoned up above of the tribes of Juda, Benjamin, and Levi, fall short of this number. The rest, who must be taken in to make up the whole sum, were of the other tribes.
[AD 585] Cassiodorus on Ezra 2:64-70