Jesus said that blasphemy against the Holy Spirit is the unforgivable sin.
Tell me, can a believer who has the Holy Spirit living in him blaspheme the Holy Spirit? Can he say that it is a demon living in himself?
The context in which Jesus made that statement was when the Pharisees were attributing to Satan the clear works of the Holy Spirit that Jesus was doing. They were stating that Jesus was demon possessed for doing miracles only God could do.
No, I don’t believe they can.
But I believe you said all your future sins are already forgiven. Yet there is that unpardonable one.
What I would conclude according to your theology as you have presented it is:
All your future sins are already forgiven, unless you commit the unpardonable one, in which case you don’t have the Holy Spirit living in you and therefore you lose the forgiveness for all your past, present and future sins that you once thought you had.