Why should it be just personal matter?

From a translated article posted on Big Blue Wave:

I do not understand why a person holding public office or engaged in political activity is obligated to renounce his Catholic identity because his party, either in the US or elsewhere in the world, imposes a moral choice based on the party's platform.

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I don't understand it either. Do we expect Bloc MPs to support a federalist cause? Or do we see many NDP MPs choosing to remain just "personally opposed" to corporate tax cuts? We don't have any Green party MPs yet but could anyone imagine Green party MPs (or MLAs) choosing to keep their passion for the environment to themselves and voting as someone else tells them to? So why should Catholics (or any other Social Conservatives for that matter) be the only ones that are expected to put their views aside when it comes to voting?