Why would the door of life be narrow at all? Why not fling it open so wide that everyone walks through without noticing? The first answer is that the commands of Christ restrict us. Eight of the Ten Commandments begin with the words you shall not. When God’s law forbids a thing, it fences us in and narrows our options on purpose.
But the rules are not the real restriction. The rules are signposts. What they point to is the character of God himself. Because God is faithful, we keep our promises. Because God is generous, there is no such thing as a stingy Christian. Because God is kind, we do not get to indulge the bad mood that curdles into cruelty. Jay Adams makes this point repeatedly in his counseling work: biblical change is never mere behavior management, it is conformity to the character of Christ.
This is why the narrow way is not arbitrary. A wide gate would require a different God. If holiness could be negotiated, God would not be holy. Peter grounds our conduct not in a rulebook but in a person: be holy, for I am holy. Walking with a holy God narrows how you live, and it should, because you are being conformed to his image.
There is a second narrowing too. There are truths we must believe. The Christian faith is not a blank canvas on which we paint whatever we like and then call it Christianity. God made the world out of nothing. Jesus is truly God and truly man. This age will end when Christ returns to judge the living and the dead. Scripture directs us to think in these ways and not others. You cannot reject the cardinal truths of the faith and still plausibly wear the name. The gate is narrow because truth is narrow.