It would be an astonishing coincidence if the one Christian we were talking about were the one secretly willing to grant religious freedom to non-Christians.

I believe, at this point, that it might be helpful to quote from "Dignitatis Humanae", an official Vatican document on the subject of religious freedom:

This Vatican Council declares that the human person has a right to religious freedom.

To elaborate slightly:

This Vatican Council declares that the human person has a right to religious freedom. This freedom means that all men are to be immune from coercion on the part of individuals or of social groups and of any human power, in such wise that no one is to be forced to act in a manner contrary to his own beliefs, whether privately or publicly, whether alone or in association with others, within due limits.

The council further declares that the right to religious freedom has its foundation in the very dignity of the human person as this dignity is known through the revealed word of God and by reason itself.(2) This right of the human person to religious freedom is to be recognized in the constitutional law whereby society is governed and thus it is to become a civil right.

It is in accordance with their dignity as persons-that is, beings endowed with reason and free will and therefore privileged to bear personal responsibility-that all men should be at once impelled by nature and also bound by a moral obligation to seek the truth, especially religious truth. They are also bound to adhere to the truth, once it is known, and to order their whole lives in accord with the demands of truth. However, men cannot discharge these obligations in a manner in keeping with their own nature unless they enjoy immunity from external coercion as well as psychological freedom. Therefore the right to religious freedom has its foundation not in the subjective disposition of the person, but in his very nature. In consequence, the right to this immunity continues to exist even in those who do not live up to their obligation of seeking the truth and adhering to it and the exercise of this right is not to be impeded, provided that just public order be observed.

Now, I'm not saying that all denominations of Christianity have an equally strong stance in favour of religious freedom (I've heard about some extremely militant modern Protestant groups, particularly in America). But this is strong evidence that there is a rather large group of Catholics who do believe in the idea of religious freedom; and if Lewis had done so as well, then he would hardly be alone in this stance.

(Dignitatis Humanae was published about two years after Lewis' death)