They don't make cover art like this too often anymore..
Light crease to centre of spine, age spotting to page block, inding tight , no inscriptions .
Panther/Granada Science Fiction, 1979, First UK printing.
An exploratory probe sent to Mars discovers an alien artifact. A follow up explorer lands and verifies that an enormous alien ship is partially buried on Mars. So a manned expedition is sent to explore this apparently abandoned alien ship. The expedition members are captured and taken underground to the habitat occupied by a combined society of humans living harmoniously with the alien Krsh.
The expedition learns that, in previous millennia, the technologically advanced Krsh were won over to the religion of the humans. This society practices Judaism but accepts Jesus as their Messiah. Originally, the Krsh had arrived at Earth on an exploratory mission. To study humans, the Krsh had offered to bring injured humans to their spaceship for medical treatment. Then, the ship was attacked by another alien species which is especially hostile and xenophobis. Even though the attack was repelled and the ship of the xenophobic aliens was destroyed, the Krsh's own ship was damaged. So they landed on Mars to hide from more potential hostility which never arrived. During the years of camouflage, the Krsh and humans crew joined together into a unified society.
Halfway through the novel, we learn that Jesus himself miraculously arrived among these people almost two thousand years earlier and had been living with them ever since.....