The kirpan is where a religious obligation meets the ordinary criminal law of weapons, and the accommodations reached make a useful case study in how states handle that collision.
The article is a blade, generally small in modern practice and sometimes worn blunted or sewn into its sheath. Sikhs hold that it is not a weapon in the legal sense, because the same code that requires it forbids its use in aggression. Legal systems have taken various approaches.
Britain provides a statutory exemption for religious purposes in its knife legislation, and Sikh police officers and soldiers wear the kirpan on duty. Canada reached the question at its Supreme Court in Multani in 2006, which held that an absolute ban on a pupil wearing a kirpan at school infringed freedom of religion where conditions could be imposed instead, and the judgment is widely cited elsewhere. India protects it explicitly in the constitution, which names the carrying of a kirpan among the rights of Sikhs. The United States has no uniform rule; the position varies by state and by institution and has produced repeated litigation over schools, courthouses and workplaces.
Air travel is the sharpest constraint and the one where no accommodation exists anywhere: kirpans cannot be carried into a cabin, and observant Sikhs check them into hold luggage.
The comparative point is that the difficulty arises from the physical form of the obligation rather than from its content. Practices requiring a person to carry something the state regulates, a blade, a substance, an animal, a document, generate legal friction that practices requiring only belief or private action never encounter, and the pattern recurs across traditions with nothing else in common.