**Answer:**

In 1809, Rammohan Roy wrote a book titled "Gift to Monotheists" in which he advocated the worship of a single God. Interestingly, this work was written in Persian.

**Importance of Persian Language:**

1. Historical Context: During the 18th and 19th centuries, Persian was a widely used language in the Indian subcontinent, especially among the educated elite. It served as a lingua franca for administration, trade, and cultural exchange.

2. Influence of Persian Literature: Persian literature had a significant impact on Indian intellectuals, including Rammohan Roy. Many renowned Persian poets and writers were highly regarded, and their works were studied and appreciated by scholars.

3. Persian and Islamic Influence: The Persian language was associated with Islamic scholarship and culture, which played a crucial role in shaping the religious and intellectual landscape of India. Rammohan Roy's decision to write in Persian may have been influenced by the desire to engage with the Islamic scholars and intellectuals of his time.

4. Reach and Accessibility: Writing in Persian allowed Rammohan Roy's ideas to reach a wider audience. Persian was widely spoken and understood by the educated class, both among Muslims and Hindus. By using Persian, Roy ensured that his message would reach a diverse group of people and initiate meaningful discussions.

5. Intellectual Discourse: The use of Persian language in Rammohan Roy's work also reflected the intellectual environment of the time. Many scholars and reformers engaged in debates and discussions in Persian, making it a language of intellectual discourse.

6. Persian as a Medium for Religious Ideas: Persian had a rich tradition of religious and philosophical texts. Rammohan Roy, being well-versed in Persian literature, may have found it suitable to express his ideas about monotheism and religious reform in a language that had a long-standing tradition of discussing such topics.

In conclusion, Rammohan Roy's decision to write "Gift to Monotheists" in Persian was influenced by the historical, cultural, and intellectual context of the time. Persian served as a means to engage with a wider audience, including both Muslims and Hindus, and allowed his ideas to be part of the ongoing religious and intellectual discourse.