He meant it. And the apostle Paul, before he was converted, was sincere too. He was a zealot when he persecuted the church of God. The basis of salvation is not that someone sincerely believes something. The basis of salvation is the work of Christ.
Sincerity counts when it is sincerity in the truth, the truth of Holy Scripture.
Now, I know that I will be called uncharitable, bigoted, and narrow-minded. The greatest danger to the Christian faith today is that many proclaim that everything is true. There are many saviors, there are many ways to peace with God. Whatever suits you ought to be good enough. Everybody ought to be able to have his own religion.