The short answer is: nobody, not Congress or the Supreme Court, has pinned down a precise definition for what constitutes corporate personhood. Proskauer partner Mal Harkins (whose Hobby Lobby article you should reference for the long answer), tells us that there are three prevalent theories about what it means to be a corporate person—and the Supreme Court's used all three of them in different contexts.

On one end of the spectrum is the partnership or derivative theory that says people have certain rights, and them banding together doesn't diminish those. On the other side is the idea that corporations are created by states, so the state can give, or withhold, whichever rights it wants. In the center is the artificial entity theory, which says that corporations exist as a matter of convenience, and we recognize their separate existence from management and shareholders, but treat them as persons in some ways. We snapped Mal pointing out a sketch a family member made of one of his arguments at the Supreme Court.

Mal with portraits his wife made of their two daughters. Not having definite lines around what it means to be a person, he says, is "sort of like driving on Route 50 and having no speed limit signs." At some point, it's important for the court to address the principle generally, not just for a corporation, but for any entity. Otherwise, we get a series of ad hoc decisions. There's a general agreement that if Hobby Lobby had structured as a partnership instead of a corporation, it would have been able to exercise those rights without challenge. If its founders had known where the lines were, they may have chosen to do so.