Religion represents a system of beliefs, understandings, and actions from which religious individuals draw an array of values for their judgments of the world. Religiousness is a multidimensional construct comprising a multitude of religious orientations of which extrinsic and intrinsic religiousness are the most prominent in scientific literature. Also, in the social psychological domain, religion represents a social group whose members show a tendency toward in-group bias, a phenomenon basically immanent in every social group. The construct of in-group favoritism refers to a tendency of favoring one’s in-group members over out-group members. The focus of this paper is oriented on the basic dimensions of religiousness and their behavioral manifestations such as in-group favoritism, prejudice, biases in prosocial behavior, etc. Based on scientific literature, the aim of this paper is to determine the relations between religiousness and in-group bias, i.e., favoring the in-group and derogation of the out-group. An additional aim is to demonstrate the importance of the influence of religious priming on the social behavior of an individual in terms of making or breaking in-group biases and out-group prejudices.