Some things about your ancestor remain the same: where they were born, who their parents were, where they died, etc. It may be difficult or impossible to determine these things, but they are facts about your relative that don’t change–even if our knowledge about them does.
Religion is one of those things that can change over time. While your own personal faith system or denomination may remain constant for your entire life, your ancestor may not have been the same. She may have been raised in one denomination, married and spent a good portion of her child rearing years in another, and died while a member of another faith.
For this reason it is advised to search records of more than one denomination in your search for an ancestor. Their faith may have been strong, but your ancestor may have spent the first fifty years of their life a Lutheran only to attend a Presbyterian church and be buried from one.