I think few would argue of the impact that Darwin and Marx had, certainly in the 19th and early 20th centuries. Darwin in particular, to shake the foundations of religion in offering a clear scientific explanation on the origins of life on Earth was no small thing. I wonder how his thoughts affected other cultures and religions though? Whether or not Darwin had any impact on the Muslim world for example – I don’t know?
I was also thinking Hitler unfortunately for the 20th century, not just for the war but for the social and political repercussions which followed it.
Extremely hard to tell, but still I wonder if there has been any significant figures this century whose influence is likely to last until the end of it or into next century? I can't think of anybody who fits that bill. Some would say Obama but I'd have to disagree with that I think.
Overall, I think it is hard to get away from Christ one way or another.