Biblical Christianity 101, a study of the book of Romans

[TheRocketSurgeon]

His point IS that because Paul believed it to the point of death, he must have believed what he wrote.

Exactly! You twisted this to:

"willingness to die" = must be true

If you were honest, you would have said:

"willingness to die" = person sincerely believes

The clear implication is that, if Paul believed sincerely what he claimed, then his claims must be legitimate, because he was committed enough to these beliefs to proclaim them in the face of potential (and actual) death.

So I feel the first version is accurate, from the point of view of someone claiming Paul had some deep insight that caused him to be willing to die for his belief. The second "sincerely believes" is simply my take on it, as a nonbeliever, in that I think lots of people sincerely believe they have deep insight into the nature and desires of God, usually because they think they have spoken to God directly, which cause them to be willing to die for those beliefs. I think Paul was either totally delusional, or else he managed to convince himself of these "truths" (no greater than the other great truths of a million other such prophets/teachers of religion, except for the fact that a lot of people have become convinced... this is no more evidence of Muhammad's actual conversation with Gabriel than it is of Paul's actual contact with Jesus by vision). 

As you point out, it need not necessarily be God that is the belief so sincere that it makes a person willing to die for that cause; it's just the specific type being discussed here. There is no effective difference between the two sayings above, when speaking with a person who thinks that Paul was willing to die because his claims really happened to him, and he sincerely held his beliefs in those events.