I wonder how narrow the gate to heaven really is? I do believe that a lot of people over the years have wondered this as well, coming to long drawn out conclusions that lead them into cults and psychotherapy. It has been a close call for me a couple times, but I am able to rest assured of a couple facts. First, Jesus is the savior of the world and that belief of what His death meant and that He did in fact rise from the dead is how we are saved by God's grace. So the next obvious thing that must be done once someone comes to this understanding is that they must obey His commands if they truly love Him. This is the tough part, because we will never be able to fully carry this out with our sinful nature that is our flesh. Don Miller gets it so right that the Christian experience is a journey and not a check list, because if it were then there would be no reason to keep it up. You would get your sharpie, make a nice looking check mark on you 3' x 5' card and enter heaven when you die. But this is absolutely not what God told us to do, in fact He told us to endure to the end, which can be interpreted as the end times and one's life.
Change is required and that is the secret to walking the narrow path to the narrow gate, and finally entering it. This is so freakin hard! But the idea of it is that we as believers in Christ can not be satisfied with how we are, because everyone falls short of the glory of God, and His law. The narrow gate will open for us only after a lifetime of living for Him, doing whatever it is that you do, but doing it for Him. That is the key. That is where so many people miss the idea and why so many will cry out "but I know you Lord, don't you know me, let me in?" And the 'master' or God will simply say, "I never knew you." People keep Jesus in their back pocket, but we need to keep Him on our mind. Yet another reference, but this is why he said that the man who thought about committing adultery is just as guilty as the one who does it, because Jesus would never have even thought about doing that since He is God, and a house divided cannot stand. Yep, that was another reference.
But simply put, there will be those who will want to deceive, but we must stay on the path that has been revealed to us, which is contained in the bible. We can only depend on other people's books and opinions as enlighteners that break the bible down for us better, cause it certainly is a hard read. That is why we must be careful to what we listen to or read about concerning God's message. We must have enough biblical knowledge as to not go astray without even knowing it, because that is what happens to so many people. We are not expected to be literary geniuses like C.S. Lewis, but we must certainly not take him as the one who we follow.
As Paul broke down in 1 Corithians 1, we are not to follow and believe in Paul, but in Jesus!