Check out this quote I found online:
"[C]onsider this fact: fewer than 20% of people actually think they are going to hell. And yet, in answering that question, Jesus says in Matthew 7 that FEW pass through the gate that leads to eternal life. 80% doesn't sound like few to me... do some of us have the wrong idea?"
There are a number of problems with this quote. First of all, you it is assuming that "few" refers to the current ratio of professing Christians to non-Christians. What's to say that Jesus isn't referring to the entire population of all the earth over all time? In that case, it's entirely plausible that 80% of people now are really Christians, as long as there are still few total Christians when all is said and done. Maybe it applies only to the people in the crowd listening to Jesus. Or, it could refer to something else entirely (as I believe).
Jesus was talking to a specific people living in a specific time. We cannot decontextualize his teachings from his audience or the culture of the time. Jesus was not talking to us when he said this, but to a crowd in first-century Israel.
And to them, he was right. For many years after Jesus said this, few entered into the kingdom. Threat of torture, death, defamation, as well as numerous rumors about Christians (e.g. they were cannibalistic, incest-practicing atheists) kept Christians in the minority.
However, Christianity grew after all. Pretty soon, it was the official religion of many states. Soon, a country (America) was founded largely on Christian precepts. Clearly, the gospel was going out to more than a few. And soon (I believe) the whole world will be Christianized.
With the "few enter" attitude, the apostles would have much reason to doubt the thousands that came into the Church at and directly following Pentecost - "Oh, I'm sure most of these are not genuinely converted." Additionally, what was the early church to make of the spreading of the gospel both East and West - they're all false believers too? Compared with then, there are tons of Christians today. Should we limit the number to about the number of Christians that existed at the time Jesus said this? Perhaps we should cap it off at a certain number (say, 144,000?). Obviously, this is absurd.
This is hard for me to say, because just last year I was teaching the same thing to my High School S.O.S. group. However, I think the Bible makes a lot more sense if we view the kingdom as a growing, conquering force rather than a remnant of only the really really committed believers.