Because ( ὅτι); for (Revised Version); "many ancient authorities read, How narrow is the gate, etc." The reading, "how" ( τί) is much easier, as avoiding the difficulty of the connexion of this verse with the preceding, but probably ὅτι is right.
The connexion is either that it is parallel to the first ὅτι, and thus gives a second reason for decision in entering through the narrow gate; or, and better, that it gives the reason for the statement in Matthew 7:13—many pass along the wrong way because the right way requires at the very outset so much determination and afterwards so much self denial.
Strait is the gate, and narrow is the way; narrow is the gate, and straitened the way (Revised Version). Not only is the gate narrow, but the way itself seems compressed ( τεθλιμμένη) by rocks, etc., on either side.
That leadeth unto life ( εἰς τὴν ζωήν). Observe, Christ does not say, "life eternal." He only cares to emphasize the thought of life in the fullest nature of life—life as "the fulfilment of the highest idea of being: perfect truth in perfect action" (Bishop Westcott, on 1 John 3:14).
And few there be that; Revised Version, and few be they that (Matthew 7:13, note). Our Lord here affirms more than the disciples ask in Luke 13:23; for there the question deals with those in a state of salvation ( οἱσωζόμενοι), here those finally saved.
Find it; i.e. the gate and all it leads to. The narrow gate is here looked at as involving life. Find. It needs a search (contrast Luke 13:13). But there is the promise of Luke 13:7, "Seek, and ye shall find."