Open source software (OSS) development is driven by its ideologies which determine its operations in an unnoticed way. Understanding ideologies of an OSS project from multiple dimensions is critical for stakeholders to make informed decisions concerning ideological issues, e.g., a commercial company may need to decide if the ideologies of an OSS project it plans to sponsor fit the company’s core values. We thus propose IdeoRate, a comprehensive assessment methodology for OSS ideologies regarding 32 themes. IdeoRate uses a project’s public information to generate quantitative and qualitative assessment for the ideologies of an OSS project. The workflow of the assessment for each theme is well defined in IdeoRate. Using IdeoRate, we conduct a multi-case study with three OSS projects, demonstrating its capability in making hidden, abstract, and vague ideologies explicit and supporting stakeholders’ decision-making. Our case study also reveals rich insights about the ideological aspect of OSS development.