Even though he's talking to a bunch of spiritual babies, the author's not gonna go back over lots of basic Christian teachings—baptism, resurrection, judgment. Instead, he'll just press ahead towards perfection. Okay, then.
For the author, being perfect means staying faithful. That means Christians need to keep the faith in Jesus no matter what hardships they're up against. It's impossible for any Christian who turns away from Jesus to come back into God's good graces. After all, Christians have heard all the good stuff about Jesus and even got a little taste of salvation. If they turn your back on those things, says the author, they're just as bad as the folks who mocked and crucified Jesus.
It's kind of a like a field on a farm. If the ground drinks up rain and grows crops, then it's in good shape. But if it starts growing stubborn weeds…well, then the only thing left to do is to burn it all down.