Is it a sin for Catholic to be a liberal, or is it a sin to be a conservative?

The question is impossible to answer because the labels liberal and conservative are too vague. The focus should be on determining whether particular propositions that individuals either hold to or reject are true or false. For example:

• Abortion is morally neutral because a person has the freedom to choose. (False)
• Euthanasia is a moral evil because it is a form of murder. (True)
• Same-sex marriage is a moral good because it allows people to enter into marriage with those they love, regardless of their gender. (False)

Care must be taken not to reduce complex human beings to the sum of the labels they bear or to unjustly condemn them on the basis of labels they either accept or are given.