I actually have more than one potential answer to my initial question now, although I'm not sure that they are exactly the original quote I’m looking for. These are the best answers so far:
The first lead comes from @DJClayworth in his comment to my original question who suggested:
At the end of one character who has been good but a
worshiper of Tash meets Aslan and throws himself down expecting to be
killed. Aslan picks him up saying "Many worshiped Tash thinking him to
be me. Many also worship me, thinking me to be Tash.
Of course I haven’t read the Narnia series so I am not very well informed about the character development or context. Here is my valiant but feeble interpretation of @DJClayworth’s comment: Perhaps CSL is conveying that the conflict between worshippers of two adversarial characters (perhaps with Aslan being Yeshua-like and Tash being Lucifer-like) are somehow worshiping the same deity. That’s the best I can possibly extrapolate so far. I don’t know what else to infer from this?
I found the actual page reference from the book providing a little more context. Here is the full paragraph from Lewis’ (1956) The Last Battle:
Then I fell at his feet and thought, Surely this is the hour of death, for the Lion (who is worthy of all honour) will know that I have served Tash all my days and not him. Nevertheless, it is better to see the Lion and die than to be Tisroc of the world and live and not to have seen him. But the Glorious One bent down his golden head and touched my forehead with his tongue and said, Son, thou art welcome. But I said, Alas Lord, I am no son of thine but the servant of Tash. He answered, Child, all the service thou hast done to Tash, I account as service done to me. Then by reasons of my great desire for wisdom and understanding, I overcame my fear and questioned the Glorious One and said, Lord, is it then true, as the Ape said, that thou and Tash are one? The Lion growled so that the earth shook (but his wrath was not against me) and said, It is false. Not because he and I are one, but because we are opposites, I take to me the services which thou hast done to him. For I and he are of such different kinds that no service which is vile can be done to me, and none which is not vile can be done to him. Therefore if any man swear by Tash and keep his oath for the oath’s sake, it is by me that he has truly sworn, though he know it not, and it is I who reward him. And if any man do a cruelty in my name, then, though he says the name Aslan, it is Tash whom he serves and by Tash his deed is accepted. Dost thou understand, Child? I said, Lord, thou knowest how much I understand. But I said also (for the truth constrained me), Yet I have been seeking Tash all my days. Beloved, said the Glorious One, unless thy desire had been for me thou wouldst not have sought so long and so truly. For all find what they truly seek.
(pp144-145). This book cited here was republished in 2007 by HarperCollins.
I have difficulty extrapolating the christian metaphor or narrative that Lewis is trying to convey. It completely escapes me. The unusual language that is almost Shakespearean doesn't help either. I can’t imagine C S Lewis’ audience of children would understand this any better than I can.
Anyways, the quote I described in my original question sounded to me more like a pointed “maxim” or “axiom” about God’s universal nature drawing all religions and human cultures from a single origin. That is more what I was getting at in my question.
Moving onto the second lead that I am working with now, there is a popular meme in circulation attributed to C. S. Lewis without a proper citation (so this destroys its credibility - - it’s almost baseless) but I think it is much closer to my original question.
One meme reads:
I am in your world,” said Aslan. “But there I have another name. You must learn to know me by that name. This was the very reason why you were brought to Narnia, that by knowing me here for a little, you may know me better there.
Another similar quote and even more axiomatic reads: “In your world, I have another name. You should know me by it.”
In this quote, I suppose it could be interpreted that the God who we know in Heaven goes by many names when humans walk the Earth and so we should come to get to know Him in the name of Christ.
At this point I suppose this meme is probably the one that I saw years ago that I can only remember vaguely today. This is as close as I’ve got to the answer to my original question but my memory is telling me that I am still missing something.
This ‘axiom’ (more like a vapid meme) is not only unsourced but it’s also way beyond the confines of Catholic doctrine which lends it even less credibility in my mind.
I checked C. S. Lewis’ Wikiquote page looking for a reference but didn’t find anything. If any of you people here on Stack Exchange have a reference, please feel free to update this answer with a correction.
I'll mark my question as answered.