The following excerpt is from the book What Is the Gospel?
Christian warns the men [Formalist and Hypocrisy] that the Lord of the city has decreed that everyone who enters the Celestial City must enter the narrow path through the gate, and he shows them a scroll he was given there, which he must present at the gate of the city in order to gain entrance. “I imagine,” Christian says “that you lack this, because you didn’t come in at the gate.”
Bunyan’s point was to show that the only way to salvation is through the Wicket Gate–that is, through repentance and faith. It’s not enough to be navigating the path of the Christian life. If a person doesn’t come in through that gate, he is not truly a Christian.”
I would heartily recommend this little book by Greg Gilbert of 9Marks. I believe that it would especially be helpful for those that have not yet entered that gate but are questioning. I also think that it would be helpful in a discipleship course.