But how do we know we are one of those until it's over?
Andrew Jeremiah's quote is typical of how many miss the understanding concerning the book of life, being motivated by the idea that each person has been individually chosen by name in Christ from before the foundation of the world. Here below in Malachi we find our first clue that this is not how that book of life gets written:
Malachi 3:16-17 "Then those who feared Yahweh spoke one with another; and Yahweh listened, and heard, and a book of memory was written before him, for those who feared Yahweh, and who honored his name. They shall be mine,” says Yahweh of Armies, “my own possession in the day that I make, and I will spare them, as a man spares his own son who serves him."
That "book of memory" and the "book of life" are one and the same book. And as Malachi reported, the names of those of God's elect who are that special possession of God got their names in that book of remembrance over time as God observed them and listened to them. And of course, if this is so then we would expect to be able to find that process of watching and listening and noticing the ways of men spoken about in the Scriptures as God goes about making that book of remembrance (aka, the book of life). And we do find that and I will show you where, but before I do I want to just say that there is also another way we know. And that other way is that Jesus said, "He that believeth on him is not judged: he that believeth not hath been judged already, because he hath not believed on the name of the only begotten Son of God." So we see that it was not until ones were able to show faith in Christ that they could have their name written into the book of life and thus not be judged. This then has to be after they are born. They could have and show that faith long before Christ's arrival but they could not have and show that faith until after they were born. Thus the book of life is indeed a book of remembrance in which God writes down those he observes and listens to and finds faithful.
Here as follows we see God doing that looking and listening and taking notes. Ask yourself if God is being partial or impartial by doing it as he says in the following:
Jeremiah 17:10 "I, Jehovah, search the mind, I try the heart, even to give every man according to his ways, according to the fruit of his doings."
Each and every human gets their reward how? "according to his ways, according to the fruit of his doings."
We play some huge tricks on ourselves with our reasoning which cause us to fail to see such simple things. We read texts like as Ephesians 1:4 "According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love", where the plural pronoun "us" indicates not the individuals but the group God predetermined would be. God in his wisdom knew he would be able to find some faithful ones to put together that group. But the only seed which was individually foreordained is the one spoken of here, as follows:
1 Peter 1:19-20 "But with the precious blood of Christ, as of a lamb without blemish and without spot: Who verily was foreordained before the foundation of the world, but was manifest in these last times for you.."
Christ was foreordained. As individuals, any of us are written into that book of memory based upon our willingness to place faith in Christ. That willingness is shown not by a vain statement that we believe, but by a belief which produces the works which prove that belief. That work is the first of the works God asks of us. And God judges us all by our works, whether our works are what he wants us to do or if our works are merely what we want to do without regard to what God wants.