Enter by the narrow gate; for the gate that leads to destruction is wide and that way is easy, so those who enter it are many. The gate is narrow; and the way is hard, that leads to life; and those who find it are few.
This is typically seen in a narrow Christian-chauvanist or judgemental spirit that seems quite inconsistent with the loving nature of God we know, not least from other things Jesus said.
There are two problems with that simple reading: This is not an isolated saying (as I've presented it here) and it is not part of a 'This is your ticket-to-Heaven' message.
Although many of Jesus' sayings make really great sound bites, they would not have been remembered and preserved that way in an oral tradition. People in oral cultures remember long passages by chunking them into coherent units.
The coherent theme behind this whole 'sermon' is not about a kingdom for dead folks; it's a message about how to join and participate in God's true Israel, the people who fulfill God's intention and enjoy the promised blessings for Israel.
The 'destruction' referred to is not God's alleged post-mortem revenge -- but a very concrete historical destruction certain to overtake Jesus' contemporary Israelites so far as they continued to seek personal wealth at their neighbors' expense while pursuing national independence by violent means. This in fact arrived about 40 years later, with the utter ruin of anyone's personal or family possessions or social status when the Romans crushed the first major Jewish revolt.
First we get a saying about how God will provide anything His children ask for. (The only problem is -- something I recently found in a commentary on Jewish ethics, searching for the phrase 'the gates of righteousness' to see how that would relate -- R. Abba bar Cahana (like Jesus in this) says that "You are His children when you are God's, that is, when you belong to God and obey Him, and devote yourselves to Him." R. Jehudah also has it that "You are children of God when you conduct yourselves as such" -- but R. Meir, speaking for the majority, instead says: "In the one as in the other case, whether they do or do not conduct themselves as such, they are and remain the children of God." While we certainly remain God's children regardless, the fact is that some ways of living fit more readily with God being able to make us happy.
Anyway, this was followed by another short piece about treating other people as you want to be treated, Jesus saying that following this principle "is" fulfilling 'the Law and the prophets.' Which leads naturally to wondering if what he says about 'not one dot will be removed from the Law' should be interpreted in that sense...
and at last we arrive at this warning. Incoherent and inconsistent? No, it makes sense together. We can enjoy the blessings promised to Israel, receive whatever we ask as God's children, so far as we do treat people the way we would like to be treated -- but there is no slack in that principle; every time we fudge on that we miss the entrance.