Notwithstanding in this rejoice not, that the spirits are subject unto you; but rather rejoice, because your names are written in heaven.
Note 4 at Lu 10:20: Jesus was instructing His disciples not to rejoice in their accomplishments for God (i.e., the demons being subject unto them) but rather in what God has done for them (their names being written in heaven). This discourages pride and is the same message as in Jer 9:23-24; 1Co 1:26-29, 4:7; and 2Co 10:17.
Note 5 at Lu 10:20: This verse, along with many other scriptural references, establishes the fact that God actually has a book in heaven in which He records the names of His true followers. This book is called "the book of life" (Php 4:3; Re 3:5, 13:8, 17:8, 20:12, and 22:19) and "the Lamb's book of life" (Re 21:27). This book is used to determine who will be delivered from the time of trouble spoken of in Da 12:1. Those who have their names written in this book will have the Lord Jesus confess their names before the Father and His angels (Re 3:5) and will be guaranteed entrance into the New Jerusalem (Re 21:27). There is a very stern warning in Re 22:19 that if anyone takes away from the words of the prophecy of the book of Revelation, then the Lord shall take away his or her part out of the Book of Life.