Religion is not a genetically determined characteristic, and I do see how he could ave been "a bit of a Christian".

You're right, it's culturally determined. You'll be surprised to learn that the culture of a mother affect the offspring even in a patriarchal society.

That said, it was a figure of speech. He was definitely not a Cristian converted to Islam and whoever thinks otherwise perhaps got confused because of the notion that his mother was a Cristian perhaps?