Richard, this is not what I believe, but rather what Lewis almost certainly believed, as evidenced by how all Christians, everywhere, throughout all history up to Lewis' time, have behaved.
I prefer to determine what Lewis almost certainly believed by looking at what he certainly wrote. The very quote that started this discussion is explicitly saying the opposite.
Besides, it's nearly five hundred years since the Thirty Years War knocked the stuffing out of Christian proselytisation by the sword, and the imperative to force people into belief, or at least practice, has been declining ever since. Further history here.
The fact that they no longer tell people to convert or die does not mean they grant freedom of religion. I'm not aware of any society with a Christian majority that has ever refrained from enforcing its moral rules on the rest of its society. I am aware of probably hundreds, if I added them up, throughout history, that have done so. Find me a dozen counterexamples and I'll listen.
Another month, another rationality quotes thread. The rules are: