Baptist is the proper adjective for Baptist.
The word for the act is "baptism" and the adjective is "baptismal". The proper noun, a name for a church or churchgoer is "Baptist".
There is no abstract noun for the proper noun 'Baptist, a word for a person. The word 'Baptist' also functions as an adjective, which is used to describe many abstract nouns; for example, Baptist beliefs, Baptist religion, Baptist doctrine, etc.
The proper adjective for southwest is "southwestern."
The proper adjective is Haitian.
The proper adjective for Portuguese is "Portuguese."
Vietnamese is the proper adjective for Vietnam.
No, old is not a proper adjective. A proper adjective is an adjective derived from a proper noun, such as Swiss cheese, Bolivian pottery, Chinese silk, etc.
The proper adjective for George is Georgian.
Mayan is itself a proper adjective.
The proper adjective for Siam is Siamese.
The proper adjective for Georgia is Georgian.
Freudian is the proper adjective for Freud.