As far as the religious thing you've mentioned Mr Bombay - Celtic was founded in Glasgow in 1887, primarily as a charity organisation to provide a means to feed the starving immigrant Irish catholic population in Glasgow. At the time there was an overbearing sense of exclusion in Glasgow against these immigrants, and they were treated as lesser people by the predominantly protestant native Glasweigans. Celtic Football Club and it's attachment to Ireland gave a sense of community and something for the immigrants to rally around. It was founded as a club open to all, and its roots in poverty and exclusion permeate through to the support of the wider Celtic family towards the cause of peoples in Chiapas, the Basque Country, Catalunya and others.
The descendents of the Irish immigrant population in Glasgow tend to have inherited a love of Celtic from their ancestors, along with their Irish catholic background.