I think that in this particular passage, Paul is referring to working to enter into the eternal rest provided by salvation and depending on the work of Christ. Other places in the Bible we are told to enter into rest on earth by letting Christ carry our burdens. In both cases, the dependency is on Jesus, and, really, both have eternal implications.
Apostasy is a sin against the Hoy Spirit. By the way, I don't believe a true Christian can fall away from Christ that completely; not because man isn't capable of that dramatic of a sin rather, Jesus promises to "keep us until that day". We are weak; he is not. If I thought for a moment that my salvation could be lost for any reason, I would lose my joy.
I do believe that there are people who don't truly enter into a spiritual relationship with Christ and, ultimately become angry with the spiritual life, ultimately denying, or more accurately choosing to go the way of sin. Having said that, we all are going to sin. Paul tells us that time and time again. We are not yet completely transformed; that which we would do what we don't wish to do; Paul teaches us that while we are saved by Christ's blood; we still live in fleshly bodies; we haven't been given our spiritual bodies yet and can be expected to sin. The wonderful thing is that, Jesus Christ died for our sins past, present and future. While no true Christian will sin "just to be sinning since I am protected", nevertheless, we are going to sin everyday and it is only through continued prayer and study of His Word that we can grow and minimize the impact of sin in our lives