Not sad--no really.
Mars may have life on it. Bacterial life. Images from a satellite above Mars excited people when they showed dark moving patches. If there is liquid on Mars, and it contains bacterial life, how will the Theists explain it away?
It's possible you mean to say -some Christians-, rather than theists? People who believe in some kind of intelligent god or gods or goddesses are not all Christians or typical in their beliefs. Life on Mars would not rule out the existence of an intelligent god, gods or goddesses. Life on Mars would mean there is life on Mars.