When did South Indians also came to identity themselves as Hindus?

 #My views are based on my limited understanding and limited to Tamil Mandalam.

Part 1 - Hindu

I will go backwards in time line.

#South Indians as Hindus in 20th Century: After world war I, that is after 1920s, eventually, any person of Indian origin (including Tamils) who did not practice Abrahamic religions came to be branded as a Hindu, thereby encompassing a wide range of religious beliefs and practices.

#North Indians as Hindus in 18th Century: Towards the end of the 18th century, the European merchants and colonists referred collectively to the followers of the Dharmic religions in Hindustan — which geographically referred to most parts of the northern Indian subcontinent — as Hindus.

#Land of Hindus as Hindustan in 13th Century:The Arabic term al-Hind, referred to the land of the people who live across the river Indus. From 13th century, the word Hindustan began to be used as a popular alternative name for India, meaning the "land of Hindus".

#People Live Across Indus River as India in 5th Century: The word Hindu or Indu was used by Greeks to denote the country and people living beyond the Indus river. Megasthenes' 'Indica' epitomizes the name for India and Indians around the 5th Century B.C.E

Hindu is the Sanskrit Word and Shindu is the Local word for River Indus.

Part 2 - Tamil Religions

I will go forward in time line.

The Tamil area had an independent existence outside the control of northern empires. The Tamil kings and chiefs were always in conflict with each other mostly over property. The royal courts were mostly places of social gathering rather than places of dispensation of authority; they were centres for distribution of resources.

# Tholkappiam between the 5th century BCE and the 3rd century CE: In Sangam poems, there is relatively meager reference given to religion in general. In the akam songs, Tholkappiar has made reference to deities in the different land divisions:

Thirumal (Vishnu) for mullai (Forest)

Murugan (Karthik) for kurinji (Mountains)

Indhiran (Indra) for marutham (Plains or Agri Lands)

Varunan, for neithal (Sea Shores) and

Kotravai (Sakthi or Kali) for palai (Desert)

#Early Religions after Sangam Period: Gradually the rulers came under the influence of vedic beliefs. Shaivite, Vaishnavism and Shaktism come into existance.

#First Five Centuries: Buddhism, Jainism and Ajvika co-existed with early Shaivite, Vaishnavism and Shaktism. It is just to mention that Ajvika survived for nearly 2,000 years through the 14th century CE in Tamil Nadu. Kalabhras period is termed as “Dark Age” of Tamil History

#6th to 13th Centuries: Saivism and Vaisnavism dominated and replaced Buddhism and Jainism. Some of the earliest temples that are still standing were built during this period by the Pallavas. Rock cut temples were the order of the day.

#13th to 17th Century: Saivism and Vaisnavism dominated. It is one of the Golden periods of Temple Architect. Damaged temples of invaders were rebuilt and new structures built and existing structures were expanded by Vijayanagara Kingdom, Madurai and Tanjavur Naickers.

#18th to 19th Century: It is a chaos in Tamil Land - full of conflicts among Local Polygars of Maduari, Tiruchi, Senji, Marathas, Nawabs, British, French.

Christianity and Islam co-existed with majority Saivism and Vaisnavism (together were over 88%). Christians are mainly concentrated in Southern districts - Kanyakumari, Tirunelveli and Tuticorin.

#Post Independence: The trauma of the partition did not impact Tamil Nadu when India was granted Independence in 1947. There was no sectarian violence against various religions. There had always been an atmosphere of mutual respect and peaceful coexistence between all religions in Tamil Nadu.