One of the great difficulties is to keep before the audience’s mind the question of Truth. They always think you are recommending Christianity not because it is true but because it
good. And in the discussion they will at every moment try escape from the issue ‘True or False’ into stuff about
good society, or morals, or the incomes of Bishops, or the Spanish Inquisition, or France, or Poland — or anything whatever. You have to keep forcing them back, and again back, the real point. Only thus will you be able to undermine
(a) Their belief that a certain amount of ‘religion’ is desirable but one mustn’t carry it too far. One must keep on pointing out that Christianity is a statement which, if false, is of no importance, and, if true, of infinite importance. The one thing it cannot be is moderately important. (b) Their firm disbelief of Article XVIII (Of obtaining eternal Salvation only by the Name of Christ).* Of course it should be pointed out that, though all salvation is through Jesus, we need not conclude that He cannot save those who have not explicitly accepted Him in this life. And it should (at least in my judgement) be made clear that we are not pronouncing all other religions to be totally false, but rather saying that in Christ whatever is true in all religions is consummated and perfected. But, on the other hand, I think we must attack wherever we meet it the nonsensical idea that mutually exclusive propositions
about God can both be true.
*Article XVIII in the Prayer Book: Of obtaining eternal Salvation only by the Name of Christ, which says 'They also are to be had accursed that presume to say, That every man shall be saved by the Law or Sect which he professeth, so that he be diligent to frame his life according to that Law, and the light of Nature. For holy Scripture doth set out unto us only the Name of Jesus Christ, whereby men must be saved.’
C.S. Lewis, "Christian Apologetics" (1945) included in God in the Dock (Eerdmans, 1970) 101-102.