What term describes excessive zeal or extreme devotion to a belief or cause?

The term that best describes excessive zeal or extreme devotion to a belief or cause is fanaticism. This word conveys not just a strong passion or commitment, but an intensity that often leads to a disregard for reason or moderation. It suggests a fervent and sometimes irrational enthusiasm for an idea, ideology, or movement, often resulting in extreme behavior or actions in support of that belief.

Devotion, while it also refers to a strong commitment, does not inherently imply that the commitment is extreme or excessive. Intensity is a more general term that can apply to the strength of feelings or actions without indicating that these feelings are based on an unwavering or extreme belief. Obsession implies a preoccupation to an unhealthy degree, but it does not specifically denote a cause or belief—it can apply to a wide range of interests, not limited to ideologies or movements. Thus, fanaticism is the most accurate term to define the scenario presented.