Not being a fully paid-up, card-carrying Christian, I may not be the most qualified, but if I had to explain 'good and positive' aspects of Christianity to a group of Martians, I would first show them the great cathedrals of Europe, and also the simple Friends Meeting Houses, the fabulous baroque churches and have them listen to Johan Sebastian Bach play the organ. I would show them the works of Michaelangelo and the great painters of the Renaissance and much, much, more. I would tell them that all this stems from one man who lived 2,000 years ago, who taught love and compassion.
I can only assume from solvitquick's question, that he would say, 'Christians are people who refuse to give blood transfusions to dying children'.