Most Christians aren't Christian?

Jesus says that many of his followers aren't going to heaven.
Read that again. Jesus didn't say atheists. Jesus said people who claimed to be his followers. He says they will be surprised because they thought they were doing everything right!
We are saved by faith in Christ! But faith means to trust. What exactly are we supposed to trust Jesus to do? What did Jesus say that he promised to do?
Please don't take my word for it. Just reflect on the Biblical passages I show you.