Intergroup relations and the factors affecting them constitute a subject of recurring interest within the academic community. Social identity theory suggests that group membership and the value we assign to it drives the expression of in-group favoritism and outgroup prejudice, among other intergroup phenomena. The present study examines how (ir)religious identities are related to topic-sentiment polarization in the form of positive in-group and negative outgroup bias during interreligious debates in YouTube commentaries. Drawing from the propositions of social identity theory, six hypotheses were tested. The data for the study, a product of a natural experiment, are comments posted on YouTube commentary sections featuring videos of interreligious debates between (a) Christian and atheist or (b) Christian and Muslim speakers. Using topic-sentiment analysis, a multistage method of topic modeling with latent semantic analysis (LSA) and sentiment analysis, 52,607 comments, for the Christian - atheist debates, and 24,179 comments, for the Christian - Muslim debates, were analyzed. The results offer support (or partial support) to the hypotheses demonstrating identity-specific instances of topic-sentiment polarization to the predicted direction. The study offers valuable insights for the relevance of social identity theory in real-world interreligious interactions, while the successful application of topic-sentiment analysis lends support for the more systematic utilization of this method in the context of social identity theory.