This bears some similarities to Dream Thief (see covers) by Stephen Lawhead. First published in 1983, it is one of Lawhead's earlier works, and is indeed heavily Christian themed (it turns out Christians and Martians worship the same supreme being).
The protagonist is Spencer Reston, a sleep researcher on a space station called Gotham. In the course of the story he goes to Mars and discovers the one of the last remaining Martians, living amid the ruins of their civilization.
he glanced up and saw a humanoid well over two meters tall watching
him from a doorway. The thing gazed at him steadily with keen
interest, its long, triple-jointed arms crossed over its narrow chest.
This fits with the description in the question, except that these Martians have triple joints instead of double ones. They voyage back to Earth and uncover a conspiracy involving another Martian, named Ortu, who had been living on Earth for millenia. The alien had originally landed with a group of others, but over the course of time the others had died:
One by one the bright ones died… rsis Atri, Pulastya, Kratu, Vasistha,
Pulacha, Marici, Angiras…' The words were familiar to Spence, who
remembered that these were the names of the gods of Indian folklore
Adjani's father had told them about. These had been Ortu's comrades,
now long dead, but still remembered and strangely revered in India's
As Spence had suspected, the coming of the Martians had given birth to
one of the most widespread enduring religions of mankind. Hinduism was
founded upon a primitive misunderstanding, a mistake of cosmic