What is a fitting description of someone who attempts to convert others to a specific belief?

The term "proselytize" directly refers to the act of attempting to convert someone from one belief or faith to another. This often involves persuasive communication and a strong intention to influence others’ viewpoints or practices. It encapsulates the idea of actively seeking to win over others to a specific cause or ideology, making it the most fitting description in the context of the question.

In contrast, "proclaim" generally means to announce or declare something publicly, without the inherent aspect of converting others. "Ameliorate" refers to making something better or improving a situation, which does not align with the concept of changing beliefs. Lastly, "infirmity" signifies a physical ailment or weakness, which is unrelated to the act of influencing or converting beliefs. Hence, the best choice is "proselytize" due to its clear connections to the idea of belief conversion.