The Lord will recount, when he writeth the peoples. The meaning is, that Zion will acquire such renown as to excite all men with the greatest earnestness to desire to be admitted into the number and rank of her citizens. It is a highly honorable condition which is spoken of, the language implying, that when God shall take a census of the people on whom he will be graciously pleased to confer the highest honor, he will write them as belonging to Zion, rather than to Babylon or any other cities; for to be one of the common people among the citizens of Zion, will be a greater distinction than to be invested with the highest rank anywhere else. We are, at the same time, taught that the cause to which we are to trace the sudden elevation of these aliens to so great honor, is the favor of God. Those who are the bondslaves of Satan and of sin will assuredly never be able to obtain, by any efforts of their own, the right of citizenship in the heavenly Jerusalem. It is the Lord's peculiar work to divide people into their respective ranks, distinguishing one from another, as seemeth good to him, all men being on a level by nature. This passage is to be understood as referring to effectual calling. God, it is true, wrote the names of his children in the Book of Life before the creation of the world; but he enrols them in the catalogue of his saints, only when, having regenerated them by the Spirit of adoption, he impresses his own mark upon them.

The Lord shall count - That is, God himself will honor those who are so born. In the previous verse, the effect of such a birth was described as securing honor from human beings. Here a higher honor is adverted to - that which will be derived from God himself.
When he writeth up the people - The word rendered "people" here is in the plural number. At the time of making an enrollment of the people, or taking an account or a census of the nations, he would mark, or cause to be marked, with special honor the man that had his birth in Zion. Out of such would his own people be taken, and those thus born would have an honor which no one else would receive from him. He would not mark with any special approbation those who had been born in Egypt, in Babylon, or in Tyre, but he would mark with special interest those who had been born in Zion. The practical truth suggested here is, that God will in the main take his people from among those who have been born in the church. As a matter of fact, while it is true that others are converted and added to the church, the great mass of church-members consist of those who have been born of Christian parents; who have been early dedicated to God; and who have been trained up for his service. See the notes at Isaiah 44:3-5.

The Lord shall count, when he writeth up the people - בכתוב עמים bichthob ammim, in the register of the people. When he takes account of those who dwell in Jerusalem, he will particularly note those who were born in Zion.
This has an easy spiritual meaning. When God takes an account of all professing Christians, he will set apart those for inhabitants of the New Jerusalem who were born in Zion, who were born again, received a new nature, and were fitted for heaven.

The LORD shall count, when he (f) writeth up the people, [that] this [man] was born there. Selah.
(f) When he calls them by his word into the Church, whom he had elected and written in the book.

The Lord shall count when he writeth up the people, Not in the Lamb's book of life; for that was written from eternity, Revelation 13:8 but in the writing of the house of Israel, among the living in Jerusalem, and with his righteous ones; which is done at effectual calling, and when admitted members of Gospel churches, whereby they openly appear to be the children of God, and are taken into the list and catalogue of saints; see Ezekiel 13:9 or in the last day, when the Lord will take the number of his people,
and cause them again to pass under the rod of him that telleth them; and will make up his jewels, complete the number of them in conversion, and collect them all together; and his counting and writing them may denote his exact knowledge of them, and his care that he lose none; but this will only concern regenerate persons; the Lord will not count nor make any account of any others, as follows:
that this man was born there; and the man that is born in Zion, even every regenerate man, will be counted and numbered by him, and declared to be his, when he makes a general survey and muster of his saints another day.
Selah. See Gill on Psalm 3:2.

The same idea is set forth under the figure of a register made by God (compare Isaiah 4:3).

When - When God, the maker and governor of this city shall take a survey of all his citizens. It is an allusion to princes or governors of cities that use to write and keep a register of all their people.