"Strive to enter in at the strait gate: for many, I say unto you, will seek to enter in, and shall not be able."  (Luke 13:24)

"Enter ye in at the strait gate: for wide is the gate, and broad is the way, that leadeth to destruction, and many there be which go in thereat:  Because strait is the gate, and narrow is the way, which leadeth unto life, and few there be that find it."  (Matthew 7:13, 14)

Read John Bunyan's: 

The Strait Gate
Or,
Great Difficulty of Going to Heaven:
Plainly proving, by the Scriptures, that not only the rude and profane, but
many great professors, will come short of that kingdom:
with directions how and why every one should strive to enter in.