Antiquity. Universality. And Consensus.
These are the three marks of truth, according to Saint Vincent of Lerins, and -among all Christian denominations- only Orthodoxy possesses all three of them, as we shall easily show in the following paragraphs:
Protestantism lacks all three of them, primarily antiquity, the first Protestant denominations dating back to no sooner than the 16th century, or -more precisely- to 1517 AD. Even the so-called Pre-Reformers did not exist prior to the 12th, 14th, or 15th centuries.
Among the remaining four1 ancient and apostolic churches, only Orthodoxy possesses universality, as we shall now demonstrate:
Catholicism is distinguished by its unique, local belief in the Filioque, completely unknown to all other ancient and apostolic churches.
Monophysitism or Miaphysitism is distinguished by its unique, local belief in the one nature of Jesus Christ, completely unknown to all other ancient and apostolic churches.
Nestorianism is distinguished by its unique, local belief in the two persons of Jesus Christ (Jesus the man, and Christ our God), completely unknown to all other ancient and apostolic churches.
Protestantism Catholicism Orthodoxy Monophysitism Nestorianism Antiquity No Yes Yes Yes Yes Filioque - Yes No No No One Nature in Christ No No No Yes No Two Persons in Christ No No No No Yes
Therefore –among all the ancient and apostolic churches– Orthodoxy is the only one that possesses catholicity, being thus "the one holy catholic and apostolic church" of the Nicene creed.
1 I did not take into account artificial hybrids, or unification movements, such as Eastern Catholicism (between Catholicism and Eastern Orthodoxy), Oriental Catholicism (between Catholicism and Oriental Orthodoxy), and Monothelitism or Monoenergism (the Maronites of Lebanon: between Eastern and Oriental Orthodoxy), all of which lack both antiquity, as well as universality.