"Enter by the narrow gate; for wide is the gate and broad is the way that leads to destruction, and there are many who go in by it. Because narrow is the gate and difficult is the way which leads to life, and there are few who find it." [Matt 7:13-14]
In my study of the book of Matthew, one thing in particular keeps jumping out at me; a common theme in many of Christ's teachings. Not all will be saved. My guess is Jesus brought this up over and over because the Jews at the time had grown complacent in their faith and thought they had a golden ticket to get into heaven because they were decedents of Abraham [Matt 3:7-12]. They relied more on their traditions and the acts of worship than on honoring God. They needed to be reminded that nobody can be grandfathered into heaven.
He presents two options: the broad way that leads to destruction, or the narrow way that leads to salvation. Many, He tells us, will enter by the broad gate that leads to destruction, either by choice or out of ignorance [Matt 7:21]. Its way is broad, and easy to follow. It is the tempting path, the one that looks comfortable, the one that we see many friends and family on. But we do not desire its destination.
Instead, we need to choose the narrow gate that leads to life and salvation. The text describes it as difficult, not because Christ's burden is heavy [Matt 11:30], but because it can be difficult going against the norm, against our friends and family who would lead us astray.
The KJV uses the word "strait" instead of difficult, meaning the same thing. This is the origin of the expression "stay on the straight and narrow," although the word has been altered to "straight" instead of "strait". It means do what's right, act with moral integrity, stay on the path that leads to life.
This passage reminds me of Robert Frost's poem The Road Not Taken in which he chooses to take the path less traveled and it made all the difference.