Tolkien was on one hand, delighted that his close friend Lewis had become a Christian. But he couldn’t get too excited because Lewis chose to join the church of England, with which Tolkien had a very bad experience with as a child.
Tolkien was a devout Roman Catholic. He had hooped that Lewis too might become a Catholic, and he was disappointed that he had returned to membership of the Church of England (the equivalent of the Church of Ireland in which Lewis had been baptised [as a child].) Tolkien was strongly unsympathetic towards the Church of England, not least because during his childhood his own mother, a Catholic convert, had been treated harshly by relatives who belonged to it – indeed he believed that this ‘persecution’ had hastened her death. As a result he was particularly sunsitive to any shade of anti-Catholic prejudice.
-Humphrey Carpenter, The Inklings, p.51
I only discovered years later that there was a lot to appreciate in the reformed faith. I was turned off from it because at 17, during my first month in college, a bible study I attended was crashed by some cage-phase Calvinists jerks. Bummer.