Augustine of Hippo
John said that books were opened, and also another book. Of the latter he did not keep silent as to what kind of book it was. This book, he says, is the book of each one's life.
Therefore, by those books which he mentioned first, we should understand the sacred books, both old and new, that is, Scripture. In these books it was shown what God commanded to be done by His commandments.
In the book that is the book of life, it is recorded what each person did or did not do from what was commanded.
If this latter book is imagined bodily, who could determine its size and length? Or how much time would be needed to read a book in which the whole life of all and each is described?
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Will there not perhaps be as many angels present as there will be men, and will each man hear his life read by the angel assigned to him?
But in that case there would not be one book for all, but a separate book for each. Yet Scripture gives us to understand that this book is one.
Therefore we must imagine a certain divine power, by whose action all a man's deeds, both good and evil, will be reproduced in memory and stand before the mind's eye with wondrous vividness.
Thus knowledge will condemn or justify the conscience, and in this way all and each will be judged together.
This divine power has evidently received the name of a book. In it, as it were, there is read what is reproduced in memory by its action.