Darwin was trained as a
clergyman. He said of his time in college "I did not then in the least doubt the strict and literal truth of every word of the Bible".
Latter on in life he gave up (it seems) on Christianity, although he did not spend too much time arguing against it (per se). This was most likely because his wife and many friends remained Christian, and he did not wish to upset them.
He wrote towards the end of his life that he had never been an atheist, but only an agnostic. This may or may not have been true. He may not have claimed to be a theologian, but he was as much as one as a man can be. Darwin most certainly saw Darwinism (such as it was at the time) as a threat to Christianity.
Darwinism is not a religion, nor did Darwin believe it to be one. But Darwin was very much concerned with religious matters and natural selections effects on them.
I would suggest that anyone interested in Darwin or the theological problems that go along with it read Created From Animals, by James Rachels.