What?! Yet it echoes other Gospel passages, like this mysterious text: "The Kingdom of God is suffering violent attack, and the violent take it by force." Paul helpfully reminds us that "our battle is not with flesh and blood, but with the principalities and rulers of this present darkness," a hint that the gate is "narrow" precisely because it is surrounded by a host of supernatural forces intent on keeping us out. Truly, strength is needed to prevail in a case like that!
On second thought, this theme of the "narrow" entrance to salvation comes up over and over in the bible. In the Old Testament, one of the wisdom themes is that of the "two ways," one leading to life, the other to death. Remember what Moses said in Deuteronomy? "Choose life, that you and your descendants may live and have a long life in the land the Lord your God is giving you." What comes next? "For the Lord is your life."
There's also the play on strength and weakness. That lead me to reread the famous story of David and Goliath. David was only a sunburned shepherd boy, hardly strong enough to prevail against a full-grown warrior! And, truth to tell, David was not relying on his own strength, though strength was precisely what he needed. "I come to you in the name of the Lord of Hosts...for God does not give victory by means of sword and spear."
Like David, Paul is not intimidated by the disproportion between human weakness and the strength needed to "enter through the narrow gate." Instead, he plays a whole game in his first letter to the Corinthians, going back and forth between "weakness and strength" and their "intellectual" incarnations, "wisdom and folly." Indeed, he boasts of his weakness! That is because the narrow gate is really the "scandal of the Cross": "a stumbling block to the Jews and foolishness for the Gentiles," but "for us on the way to salvation Christ the power of God and the wisdom of God."
Just today, Pope Benedict just commented in the same vein:
"The sword St. Paul holds in his hand also recalls the power of truth, which can sometimes wound and inflict pain, The Apostle remained faithful to this truth unto the end, ... he gave his life for it. The same logic holds true for us, if we wish to bring the kingdom of peace announced by the Prophet Zechariah and achieved by Christ: we must be ready to pay in person, to suffer ... misunderstanding, rejection and persecution. It is not the sword of the conqueror that builds peace, but the sword of those who suffer and give up their own lives" (from Vatican Information Service).