Anguttara Nikaya. V,194

The wanderer Uttiya asked the Lord: 


“If, with full comprehension, the good Gotama teaches Dhamma to his disciples for the purification of beings, 

for the overcoming of sorrow and despair,
for the ending of grief and dejection,
for reaching the method,
for the attaining of Nirvana,

then will the whole world attain it, or half of it, or a third?”

At these words, the Lord was silent.

Then Ananda thought: “Uttiya must not be allowed to think that the Lord cannot answer this all-important question.”

So Ananda said: 
"I will give you a simile Uttiya. Imagine a walled town with strong foundation and towers and a single gate. At the gate a watcher who is clever and alert who lets in known people and keeps out strangers. As he patrols the walls he would not see a hole in the wall big enough for even a cat to slip through. And therefore he would know that whatever creatures big or small enter or leave the town, they all do so by the gate.

It is the same with the answer to your question, a question that is not important to the Lord. What the Lord says is this. Whoever has escaped, is escaping or will escape from the world, they will do it by abandoning the five hindrances, those defilements of the mind that weaken wisdom; they will do it with mind well-established in the four foundations of mindfulness, and by developing the seven factors of enlightenment.”