In recent decades, there has been a revival of contentious social movements worldwide. Given the collective action problem, what factors motivate ordinary citizens to join protests? If the issue of a protest matters, what factors motivate people to weigh different issues differently? This paper devises a unified theory of collective action and argues that individuals are more likely to join a protest when they perceive higher values of the public good, higher expressive benefits, higher expected turnouts, and lower costs of participation. It contends that the effects of those factors are moderated by individuals’ post-materialist values and confidence in future personal freedom. Employing an online conjoint survey experiment to analyze protest participation among citizens in Hong Kong, this paper finds that protesters are more driven by liberal-democratic issues than socioeconomic issues. The findings highlight the mechanisms through which political beliefs increase protest participation. Political beliefs change the effects of public goods associated with certain issues, and to some extent alter the cost-benefit calculations of potential protesters.