Psalms 87:6 Gill

The Lord shall count when he writeth up the people, Not in the Lamb's book of life; for that was written from eternity, Rev 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 Eze 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 Psa 3:2.

Spurgeon writes...

Psa 87:6

“The Lord shall count, when he writeth up the people, that this man was born there.” At the great census which the Lord himself shall take, he will number the nations without exception and make an exact registry of them, whether they were by their natural descent Babylonians or Tyrians, or other far-off heathen. May it be our happy lot to be numbered with the Lord's chosen both in life and death, in the church-roll below, and in the church-roll above. Jehovah's census of his chosen will differ much from ours; he will count many whom we should have disowned, and he will leave out many whom we should have reckoned. His registration is infallible. Let us pray then for that adoption and regeneration which will secure us a place among the heaven-born. It was thought to be a great honour to have one's name written in the golden book of the Republic of Venice, kings and princes paid dearly for the honour, but the book of life confers far rarer dignity upon all whose names are recorded therein.