My question is about the spirits in the pre-mortal spirit world. Will God wait for all spirits in the pre-mortal world to be born before the second coming?
D&C 45 describes the events surrounding the Second Coming of Jesus and the ushering in of the Millennium. During the 1000 year reign of Christ, “the earth shall be given unto them [that withstood the burning of the second advent] for an inheritance; and they shall multiply and wax strong, and their children shall grow up without sin unto salvation.” (D&C 45:58). It could be argued that “their children” is referencing the children who are alive at that time. In that case, the Millennium will be poorly named indeed (how can you fill out 1000 years with a single generation?), and worse, the very children who “grow up without sin unto salvation” are the ones that rebel and launch the battle of Gog and Magog.
The simple answer is that children will continue to be born during the Millennium. Brigham Young taught:
“The Millennium consists in this—every heart in the Church and Kingdom of God being united in one; the Kingdom increasing to the overcoming of everything opposed to the economy of heaven, and Satan being bound, and having a seal set upon him. All things else will be as they are now, we shall eat, drink, and wear clothing.” (Teachings of Presidents of the Church: Brigham Young, The Last Days).
In a revelation to Joseph Smith the Lord said,
‘In that day an infant shall not die until he is old; and his life shall be as the age of a tree;
and when he dies he shall not sleep, that is to say in the earth, but shall be changed in the twinkling of an eye, and shall be caught up, and his rest shall be glorious’ (D&C 101:30-31). (emphasis mine)
Having children is one of those things that will be as they are now, except that they will be reared in a purer environment.