If Mark believes in Jesus and is born again then he is a christian.
He can be a christian and still be wrong about Jesus being the only way to be saved, this might be possible, but it's strange. The bible makes it clear.
If Mark is a born again christian then it's cool. I would be happy to hear he is a born again christian.
I am also happy to hear that Stapp is a christian.
I was never really sure about Stapp cause on the Creed website it also said something like "who are we to say that only christians will be saved..." and this stuff was written in the name of Stapp. But Stapp did never write this. Somebody else did and put it on the homepage. Stapp was not even aware of it.