House Law and Religion
The Princely House of Liechtenstein revised Constitution of 2003 is not as strict as people may imagine.
The only mention of religion or faith in the entire document is in the Preamble, which states:
For several centuries the Liechtenstein family has been governed by statutes.
In parts the old provisions no longer met modern requirements. The family
therefore decided on 26 October 1993 to repeal the previous statute and to
adopt a new one in keeping with its traditions. These centuries-old family
traditions include the Catholic faith, which shall also serve as a guiding
principle in future decisions, whilst respecting the freedom of belief and
conscience of the individual.
Article 7 covers marriage and there is no mention of religious requirements:
Marriage
1) Where a member of the Princely House has the intention to marry, he must
inform the Reigning Prince thereof and deposit with the secretariat of the
Reigning Prince all documents required under Liechtenstein law, together with
a written and unconditional declaration by the other party to the intended
marriage that that party recognises the binding nature of all aspects of this
Constitution, for that party and all issue arising from the intended marriage...
If [the documents] are complete and the Reigning Prince considers that all
conditions for approval of the intended marriage are met, he shall declare
that he consents thereto. The Reigning Prince shall then notify all members
of the Princely House who are of full age.
2) [Members of the Princely House have one month to object to the intended marriage in writing]
Such objection shall be admissable only insofar as it is based on the lack of
capacity to enter into mariage on the part of one of the parties to the
intended marriage, or the existence of any impediment to the marriage or
on the ground that the marriage would be detrimental to the reputation,
esteem or welfare of the Princely House or of the Principality of Liechtenstein.
The Reigning Prince shall give a ruling on the objection, after hearing the
parties to the intended marriage.
Prince Rudolf is currently placed about 13th in the line of succession to Prince Hans Adam so I doubt there would have been any great difficulty
in gaining the Reigning Prince's formal approval.