The solar myth theory of Christianity, as I understand it, was first described by Gerald Massey in the 19th century in his book 'Ancient Egypt, Light of the World'.
A good summary of his work is found in the book 'Gerald Massey's Lectures'. This was extended by Thomas Doane in his book 'Bible Myths' to include other religions such as Hinduism, Buddhism (assuming it to be a religion) and pagan religions.
The theory is based upon rituals concerning the passage of the sun in the skies of the Northern Hemisphere. Hence Christ is born 3 days and 3 nights following the winter solstice, John the Baptist at the summer solstice, Easter is connected to the Vernal Equinox, Lady Day to the Autumnal Equinox. The comparisons between Christianity, Hinduism and Buddhism are most striking in a long list of tables given by Doane. The extensive references to the works of other writers in the book seem to prove that much debate was raging in the 19th century as to origin of religion. This type of debate seems to have been forgotten in favour of political correctness today.