Step 1: Understanding the Concept: 
The question asks to identify the sociologist who gave a foundational theory for the "Sociology of Religion" as a distinct field of study. 
Step 2: Detailed Explanation: 
While several sociologists have studied religion, two are considered central to the founding of its sociological study: 
- Max Weber: He wrote extensively on religion, most famously in "The Protestant Ethic and the Spirit of Capitalism," where he analyzed the relationship between religious ideas and economic development. This is a sociology \textit{of} religion. 
- Emile Durkheim: In his work "The Elementary Forms of the Religious Life," Durkheim proposed a general theory of religion itself. He argued that the essence of religion is the worship of society and that religious rituals create social solidarity. This work is considered a foundational text that established the functionalist approach to the sociology of religion. 
Both Weber and Durkheim are correct answers in a broad sense. However, Durkheim's work is often seen as providing a more universal and systematic sociological theory of what religion \textit{is} and what it \textit{does} for society, thus being a more direct answer to "the theory of 'Sociology of Religions'". 
Step 3: Final Answer: 
Emile Durkheim is a key founder of the sociology of religion, known for his theory on the social functions of religion.