Myths, Lewis told Tolkien, were "lies and therefore worthless, even though breathed through silver."

"No," Tolkien replied. "They are not lies." Far from being lies, they were the best way sometimes the only way of conveying truths that would otherwise remain inexpressible. We have come from God, Tolkien argued, and inevitably the myths woven by us, though they contain error, reflect a splintered fragment of the true light, the eternal truth that is with God. Myths may be misguided, but they steer however shakily toward the true harbor, whereas materialistic "progress" leads only to the abyss and the power of evil.

Building on this philosophy of myth, Tolkien explained to Lewis that the story of Christ was the true myth at the very heart of history and at the very root of reality. Whereas the pagan myths were manifestations of God expressing Himself through the minds of poets, using the images of their "mythopoeia" to reveal fragments of His eternal truth, the true myth of Christ was a manifestation of God expressing Himself through Himself, with himself, and in Himself. God, in the Incarnation, had revealed Himself as the ultimate poet who was creating reality, the true poem or true myth, in His own image. Thus, in a divinely inspired paradox, myth was revealed as the ultimate realism.

Links: https://www.catholiceducation.org/en/culture/art/j-r-r-tolkien-truth-and-myth.html

Link: http://www.tolkien.ro/text/JRR%20Tolkien%20-%20Mythopoeia.pdf