It is interesting that Lewis actually became convinced of the existence of God through his talks with Tolkien. It was disappointing to Tolkien, a devout Catholic, that Lewis chose to affiliate himself with the Church of England. Lewis and Tolkien had some serious theological differences. Tolkien felt Lewis had settled for less than the whole truth in acception Christianity but not Catholicism. I'm sure Lewis marriage to a divorcee was quite distressing to him.
As far as philosophical differences concerning writing, I do know that Tolkien was a purist where myth was concerned. He was deeply offended, not only by allegory itself, but by the very idea that Father Christmas could be in the same story as a faun. Lewis, on the other hand, said that since his stories were for children and children tended to mix characters in their own pretending that he felt it was entirely appropriate.
Giving all due respect to Tolkien, I don't think he was able (or necessarily intended) to avoid expressing his own faith in his work, he simply wasn't expressing it allegorically. Tolkien's traditional Catholic beliefs about good and evil, temptation, divine (or supernatural) intervention, and women to name a few are intwined in the mythology he created. He intended to tell a great story and to create a new mythology to go with the language he had already created. While his deliberate intention of convincing Lewis of God's existence reflects a concern to share his faith, I think he has a less overt evangelist. His beliefs about what is true about how the universe works, the divine, morality, good and evil are reflected in his writing as an unlying part of the mythology he created. What is true or reality in Middle Earth is that way because of what Tolkien as creator believed. Tolkien claims it wasn't deliberate, perhaps not, but his creation flows from who he is.
Lewis, on the other hand, intentionally expressed his faith. His motive was to tell a great story and to use it to evangelize. Having both motivations doesn't make him less of a writer, just less complicated. Lewis lays his heart out there for the reader. Tolkien's works requires the reader to strip off layer after layer to get to it's writer's heart.
I love them both.
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Do justly, love mercy, walk humbly with your God. Micah 6:8
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