The author of the book of First John is not identified within the text of the book but there is significant evidence to suggest that John was the one who penned this book. This book is written in a thematic pattern rather than a linear progression.
One reason this book was written was for the purpose of fighting false doctrine. This book was also written to reassure Christians that we are indeed saved. In order to have this kind of assurance we must make sure our beliefs are in agreement with the Scriptures. We also need to spend time studying the promises of God.