The experimenters explored how religious fundamentalism related with religious orientation, irrational thinking, and immature defense mechanisms. They also explored the possible moderational role of the Big 5 personality factors. The participants were predominantly Greek Orthodox College students from a Cypriot University. The experimenters employed a cross-sectional design and required participants to complete a series of self-report measures. Religious fundamentalism significantly predicted irrational thinking. Intrinsic and personal extrinsic religious orientations significantly predicted religious fundamentalism. The results provide support for the idea that the more dogmatically one holds their religious beliefs, the more likely they are to think irrationally.