When I was younger I thought of the narrow gate as the difficulty in keeping God's stringent commandements. It's narrow because he's got a tough standard.
Then, as I matured, I began to think the gate was narrow and few find it because it consists of forsaking dependence on self and relying on the grace of God.
This way is narrow because it goes against the human grain. In the religous realm we often ask "what can I DO?" We lean hard on our own efforts and wits. But the real way into God's wider realm consists in ceasing from action and finding rest in god's grace in Jesus.
Now in my later middle age I have come to discover a third interpretation, the most practical and perhaps the most Christian of all. The narrow way is simply the Golden Rule: do unto others as you would have them do unto you.
This seemingly simple guideline to behavior not only summarizes the Law and the Prophets, it also provides a sure path to holiness and sainthood. The daily grind of asking about and acting upon what will be best for those around me, this is the pumice with which God buffs our stony hearts until they shine. The vocation of living "for the other" is the truest way to self fulfillment, and it takes us right down the stony path our Savior trod.
I hear again today the words of the Prologue to St. Benedict's Rule:
Saint Benedict (480-547), monk
The Rule, Prologue
"The Lord, seeking his laborer in the multitude to whom he thus cries out, says again, "Who is the one who will have life, and desires to see good days?" (Ps. 34:13) And if, hearing him, you answer, "I am the one," God says to you, "If you will have true and everlasting life, keep your tongue from evil and your lips that they speak no guile. Turn away from evil and do good; seek after peace and pursue it" (Ps. 34:14-15)…
What can be sweeter to us, dear brothers, than this voice of the Lord inviting us? In his loving kindness the Lord shows us the way of life. Having our loins girded, therefore, with faith and the performance of good works (Eph. 6:14), let us walk in his paths by the guidance of the Gospel, that we may deserve to see him who has called us to his Kingdom (1 Th 2:12).
For if we wish to dwell in the tent of that kingdom, we must run to it by good deeds or we shall never reach it. Let us ask the Lord, with the prophet, "Lord, who shall dwell in your tent, or who shall rest upon your holy mountain?" (Ps. 15:1)
After this question, brothers, let us listen to the Lord as he answers and shows us the way… And so we are going to establish a school for the service of the Lord. In founding it we hope to introduce nothing harsh or burdensome.
But if a certain strictness results from the dictates of equity for the amendment of vices or the preservation of charity, do not be at once dismayed and fly from the way of salvation, whose entrance cannot but be narrow.
For as we advance in life and in faith, our hearts expand and we run the way of God's commandments with unspeakable sweetness of love (Ps. 119:32). Thus, never departing from his schooling but persevering in the monastery according to his teaching until death, we may by patience share in the sufferings of Christ (1 Peter 4:13) and deserve to have a share also in his Kingdom."