Christianity in the roman empire.

[Theodosius the Great]

IIRC recent archaeology has revealed pre-Constantinian Christian uses of the Chi-Ro. The only one I know of is from the Vatican Necropolis in the tomb of the Valerii which dates to Marcus Aurelius' reign. Here's a recent article about the tombs beneath Saint Peter's Basilica : http://www.foxnews.com/story/0,2933,358641,00.html

Here's mention about the Chi Ro inscription : http://www.saintpetersbasilica.org/Necropolis/TombH.htm

If it was already a Christian symbol at that time then Constantine merely popularized it along with Christianity.

Somehow I doubt that the Christians consciously 'took' it from the cult of the sun god which wasn't even invented until the late third century AD and probably never gained as many adherents as Christianity.

~Theo