I have a number of sermons on two of these passages that we have posted on the internet in Real Audio format. IMO, the subject of Hebrews 6 actually begins earlier (5:12ff). You can start at this link and then listen to as many of the sermons that you think will be helpful. IMO, it is the job of the Holy Spirit to grant assurance, so I'm not prepared to say that you should not have any doubts about your salvation. But I will say that the doubts should lead you back to the cross of Christ.
