Explanation:

- He is an excessively enthusiastic and strict follower of Judaism: This describes someone who is intensely passionate and potentially uncompromising in religious devotion.

- Option 1: A janissary - Historically, janissaries were elite soldiers in the Ottoman Empire. The term doesn’t relate to being a follower of Judaism.

- Option 2: A Zionist - A Zionist supports the re-establishment of a Jewish homeland, particularly in Israel. While related to Judaism, it doesn't necessarily imply excessive enthusiasm or strictness.

- Option 3: A zealot - A zealot is someone who is fervently devoted to a cause or religion. This matches the description of being excessively enthusiastic and strict, making it the most appropriate substitution.

- Option 4: A healer - This refers to someone who cures others, which is unrelated to being a follower of Judaism.