Although extensive research has documented the effectiveness of common or dual in‐groups on improving intergroup relations, little is known about how individual‐difference variables affect people's willingness to make such re‐categorizations in the first place. Here, we demonstrate that individual differences in religious fundamentalism predict willingness to categorize in terms of the common Abrahamic religious origins of Christianity and Islam among Christians and Muslims. Study 1 (n = 243 Christians, 291 Muslims) uses multigroup structural equation modeling and Study 2 (n = 80 Christians) an experimental manipulation to show that religious fundamentalism causes lower dual Abrahamic categorization, which, in turn, predicts more positive attitudes toward the respective out‐group, mediating the negative effects of religious fundamentalism on religious intergroup bias. While making the general case that individual differences may play important roles for dual categorizations, these results also highlight the specific positive potential of dual ecumenical categorizations for improving interreligious relations. Research and societal implications are discussed. Copyright © 2014 John Wiley & Sons, Ltd.