"Knock, and it shall be opened unto you." Matthew 7:8
The iinscription above the Wicket Gate through which Christian must enter reads, "Knock, and it shall be opened unto you." Bunyan makes it very clear that only through this gate can one enter to begin a valid journey to the Celestial Kingdom.
"He that will enter in must first without
Stand knocking at the Gate, nor need he doubt
That is a knocker but to enter in;
For God can love him, and forgive his sin."
Jesus says in Matthew 14:
"Enter through the narrow gate. For wide is the gate and broad is the road that leads to destruction, and many enter through it. But small is the gate and narrow the road that leads to life, and only a few find it."
Just as Christian entered through the narrow gate to find salvation, so Bunyan contemporaries could only find salvation through Jesus Christ.