The club was started by working class Irish immigrants and a lot of their support still comes from the descendants of those people. I read an interesting article recently about how the club winning the European cup back in the 1967 did a lot to improve the confidence and status of their supporters.
Rangers too have a large working class support but as many are staunch unionists I wonder what their reaction would have been in a similar situation.
Of course the answer to the question "Do you support Rangers or Celtic" ?. Is - I am an atheist I support Partick Thistle.

This has nothing to do with Catholic/Protestant sensitivities..this is about a loathing of all things Tory. These Celtic fans obviously felt a sense of outrage that one of their biggest fans had shown support for a hated figure who represents to them upper class Tory privilege and who in the past has shown his utter disregard and disparagement for all things Scottish. The ex Hibs BBC guy was right, people should leave their politics at home when they go to support their football team, but just as they take their religion with them in Scotland, their politics evidently comes too. Probably wiser that he’d kept his mouth shut, but he’s no stranger to the passion which all three...football, religion and politics raises in the Scots, being one himself.