Do you have a link?
I ran across that surprising fact a few years back when I was researching Sirhan's motive for the assassination. A friend of mine worked with Sirhan one summer and knew him, but Sirhan was quiet and never spoke of politics. There's a lesson there IMO, for those who think we can weed out potential terrorists, but that's another matter. Google "Sirhan Sirhan Christian" and you'll get a number of hits. Here's a sample:
http://www.danielpipes.org/comments/1928
http://www.maebrussell.com/Articles%20and%20Notes/Sirhan%20Demands%20Retrial.htm
http://en.wikipedia.org/wiki/Sirhan_Sirhan
http://crimemagazine.com/04/bobbykennedy,0527.htm