Make every effort to enter through the narrow door, because many, I tell you, will try to enter and will not be able to.
I've been white water rafting several times on the New River in West Virginia. (I even have video of me swimming the end of a class 5 rapid, while everyone else stayed in the raft.)
One of the rapids is calling Thread the Needle. It's likely just what you picture - two large rock columns in the middle of the river, with rapids going between and around them. When the conditions are right, you can float on your back through the rapid. The goal is to go easily between the two rocks, but it isn't quite that easy.
When I read this verse today, I thought about this rapid and the parallels it provided.
1. You can't go through the narrow middle unless you first choose to get into the river's flow. The guide simply offers this as an option. Some people stay in the raft and go around.
2. A life jacket is a neccesity!
3. You are in the river and on your own. This is no team effort this time. In fact, there is no effort at all - you just lie back, keep your feet pointed in the right direction and enjoy the ride.
Lastly, even though everyone starts in the same river headed the same direction, not everyone will thread the needle. It makes a difference where you are in the current's flow. In order to go through the narrow middle, you have to position yourself in the current that goes through that way. If you aren't paying attention, you may end up in a current that steers you away from the middle or may even crash you right into one of the rocks.
Jesus Christ is the needle threading rapid. Got your life vest on? Feet pointed toward Him? It's an adventure for eternity.