I guess it all depends on what we mean when we say "believe". Do we mean a mental assent - like I know in my brain that it is true - or do we mean that we have faith in (trust in) God?
We also have so many terms that we throw around... "saved" "born again" "sanctified" etc. that sometimes cause confussion in this issue, IMO.
I have always believed that the moment a person believes and confesses, they are saved and born again. We who were dead in our sins are buried with Christ and made alive again - given regenerated spirits. Although it may not be something we can see and it may not be something we can really feel (or the feeling of it comes and goes)... it's as real as our natural births.
So my question would be... if this supernatural act of regeneration happens, and it happens by God's act, not our own (by Grace through faith, not by our works).... then how could a time of disbelief or doubt on our parts make us dead again?
If a person is truly born again, IMO, it may be possible for him to willingly and knowingly reject Christ. But I don't believe that it's possible for him to backslide to a point of non-salvation. And I believe that the Holy Spirit is going to continue to call to the person to bring him back to righteousness.
If a child of God is being disobedient, God doesn't cut that person off. God disciplines him to bring him back. And if God doesn't discipline him, then he's probably not really a child of God. God chastens those He loves.
To me, this is comforting. God loves me enough to not give up on me and let me go my own way... He might make it very painful to me, but in the long run discipline that was painful works in us a much better result.