I think that page is misleading. They ignore the fact that when Jesus was asked whether few will be saved, his immediate answer was, in effect, the question is whether *you* are saved.
Jesus used "many" quite freely, for all kinds of people, saved and unsaved. You can't take one of the passages warning that many are going the wrong way and say that therefore most people are damned. After Jesus talked about it being hard for the rich man to be saved, his disciples were alarmed. Jesus answer was "what is not possible for men is possible for God." To me that leaves us with hope that God may still help those who can't find the narrow gate.
While they never quite say it, that passage leaves the impression that pretty much everyone other than them is in danger because of theological errors. There's no sign in Jesus or Paul that this is the case. Paul based salvation on faith, Jesus on a life based on love, shown in service, but with repentance covering failures. There is simply no sign that errors about baptism, the millennial rule of Christ, or even legalism (the Lordship Salvation folks) imperils the salvation for someone who trusts in Christ.