Too many flounder from spiritual high to spiritual high...yet are never firmly established in the faith.
They smile at the right time, are fluent in Christianese, maybe even share their "faith," yet they tremble when they read Matthew 7.21, "Not everyone who says to Me, 'Lord, Lord,' shall enter the kingdom of heaven."
The book of First John has a mission statement..."I write these things to you who believe in the name of the Son of God that you may know that you have eternal life." (1 Jo 5.13)
Sadly many are trained to take people to that verse after they "receive" Christ in order to let them "know" that they are saved...but the "counselor" doesn't make the time to help the counselee begin to understand what "these things" are (which, of course, is everything else that comes before 1 John 5.13).
So, if you, or someone you know, struggles with assurance, point them to 1 John.
And buy a copy of this book to help. The title will be a turn-off to many (in fact, Paige Patterson - president of Southwestern Baptist Theological Seminary - who wrote the forward to the book says, even in recommending the book, that "I confess that I dislike the title...").
At only 120 or so pages, it is not lengthy, but is loaded with scripture, common sense, and honesty.
I recommend it highly...and if you buy it by clicking on the following link you'll help support the Hager family...who serve as home missionaries