Without appearing to flog a dead horse this post is made "Just for the record."
The CNI is a Certificate of no impediment (sometimes called Legal Capacity to Marry)
This certificate provides evidence to officially authorised people that the marriage will be recognised as a valid marriage in both Philippines and the country of the foreign spouse.
A valid CNI is required by Philippine law.
The Foreign and Commonwealth Office states:-
You should check with the authorities in the country where you are marrying whether they can accept the CNI as issued by the UK registrar or if it must be exchanged for a locally acceptable version (ie. a certificate in the local language) issued by the British Embassy or Consulate
From Article 21 of The Family Code of The Philippines:-
When either or both of the contracting parties are citizens of a foreign country, it shall be necessary for them before a marriage license can be obtained, to submit a certificate of legal capacity to contract marriage,
issued by their respective diplomatic or consular officials.