Validity of an Arya Samaj marriage certificate. Is marriage legal?

Validity of an Arya Samaj marriage certificate. Is marriage legal?

Yes the marriage is valid.
If you got married under the Arya Samaj Mandir then get this marriage registered under section 8 of the Act by the Marriage officer/Registrar of the district/state, where this marriage took place.

After 2006, orders of Supreme Court are strict on the issue of registration of marriages in personal laws. An Arya Samaj marriage can be registered either under the Hindu Marriage Act, 1955 or under the Special Marriage Act, 1954. The Hindu Marriage Act is applicable in cases where both husband and wife are Hindus, Buddhists, Jains or Sikhs or where they have converted into any of these religions. Where either of the husband or wife or both are not Hindus, Buddhists, Jains or Sikhs, the marriage is registered under the Special Marriage Act, 1954.
Hindu Marriage Act places a responsibility upon the states to make laws regarding registration of the marriage but also places an emphasis that where a marriage is not registered, its non registration will not invalidate the marriage. It is often advised to get the marriage registered as per the Supreme court orders and, it is the correct way to avoid disputes in future regarding breakdown of marriage.
Registering Arya Samaj Marriage as per Hindu Marriage Act

Registration is done with any Sub-divisional Magistrate, offline. Another option available at few jurisdictions like Delhi is of online registration. An appointment comes after 15 days.
Fulfilling the formalities of registration form.
Any document that provides the date of birth of the individuals.
2 passport size photographs of both the parties are required, also one marriage photograph and Marriage invitation card (optional).
Attestation of gazetted officer is mandatory
After fulfilling all the formalities, it is the duty of the district court to provide the couple a marriage certificate.
A marriage registration cost around INR 100- 200.