What term describes a person who strongly supports an idea or cause to the point of fanaticism?

The term that describes a person who strongly supports an idea or cause to the point of fanaticism is "zealot." A zealot is someone who possesses an excessive or militant enthusiasm for a particular belief or ideology, often leading to unwavering commitment that may disregard other perspectives or alternative views. This term conveys the intensity and sometimes extreme nature of their support, which can manifest in various contexts, such as politics, religion, or social issues.

In contrast, the other options do not capture the same meaning. "Voracious" typically refers to having a very eager approach, especially towards consuming food or information, but it does not imply fanaticism. "Vex" relates to causing annoyance or frustration, which is unrelated to fervent support. "Xenophobia" describes a fear or dislike of people from other countries or cultures, rather than a passionate support for a cause. Thus, "zealot" is the most appropriate choice for describing someone with fervent and potentially fanatical support for an idea or cause.