Matthew 7:12-13 reads as follows. "Enter through the narrow gate; for the gate is wide and the road is easy that leads to destruction, and there are many who take it. For the gate is narrow and the road is hard that leads to life, and there are few who find it." (NRSV)
Let's clear away one matter upfront. This passage is often used in American evangelical and fundamentalist circles to argue that Christianity is the only way to salvation. Matthew 7:12-13,taken in context, is not concerned with such a question. Instead, the passage fits neatly into a strand of Jewish tradition and also serves as the opening volley in a concluding challenge.
For example, when I read the passage I am reminded of Psalm 1: "Happy are those who do not follow the advice of the wicked, or take the path that sinners tread, or sit in the seat of scoffers; but their delight is in the law of the Lord, and on his law they meditate day and night." The idea of two ways, one leading to life and the other destruction, has deep, old roots. The Christian tradition absorbed the image, first from Jesus then through various reformulations, ranging from the Didache to The Pilgrim's Progress. It even crops up in The Lord of the Rings, when Galadriel warns Frodo that the quest walks on the sharp edge of a knife, in other words on a very narrow road.
The tradition assumes human choice. We decide which way to take. In my view, this is not so much a single decision as a series of decisions. We choose who to take seriously, who to believe and so set the course of our lives. Our choices shape us. Over time we become more and more nearly like that upon which we focus. That being so, Jesus' warning and promise is apt.
A related question arises: if our lives are shaped by the road we choose to walk, by that to which we pay attention, what is it we're called to take seriously? Context helps answer the question. The two of us have covered much of the content. It's summarized to a great extent in the golden rule. The Beatitudes and the Lord's Prayer contribute additional particulars. If I had to sum it up in a sentence, it would go something like this: "The way of life is to love God, neighbor and yourself well and take the consequences!" Based on our past interaction, I would guess you might prefer a phrase that employs terms such as justice and compassion.
Jesus challenges his listeners to take the matter seriously, as something of transcending importance. In subsequent verses, he will illustrate some of the challenges faced by those who do so: false teachers, false self-perceptions and the like.