Step 1: Identify the quote.
The statement One caste, one religion, one God for mankind is a powerful philosophical teaching that advocates universal human equality and firmly rejects caste-based discrimination and social division.
Step 2: Connect the quote to its author.
This message is the famous teaching of Sri Narayana Guru, the revered saint and social reformer of Kerala. In Malayalam: Oru Jathi, Oru Matham, Oru Daivam, Manushyanu. He uplifted oppressed communities and championed social equality throughout his life.
Step 3: Eliminate other reformers.
Vakkom Moulavi, Ayyankali, and Thycaud Ayya were also great reformers, but this specific quote belongs to Sri Narayana Guru.
\[ \boxed{\text{Sri Narayana Guru}} \]