The basis of the concept of combating terrorism lies in understanding the causes and conditions that lead to terrorism. Knowing the causes and conditions that constitute terrorism will be effective both in understanding terrorism from a logical point of view and in determining the correct strategies to combat it. The main areas of terrorism, based on the falsifying interpretation of religious doctrines, feed on religious motivation, religious justification based on religious judgments, an organization based on believers, and ostensibly based on religious worldviews. For this reason, the terrorism in question is usually carried out by those who are trying to exploit religion in these areas, to provide religious motivation, and, as the real purpose, to terrorize. These groups engage in extreme deception that falsifies religious texts. In addition, the groups in question make takfir and condemn individuals or communities with their distorted versions of religious facts and events. Therefore, fundamentalist/radical ideological thought is an important source of motivation for participation in terrorist organizations, but this ideological infrastructure in a way constitutes the first step of the process of joining terrorist organizations. Within the framework of this information and determinations, our study is limited to the terrorist organization ISIS, which has a religious-radical ideology, which makes harsh judgments without interpreting religious texts, treats religious texts with a strict perspective, and foresees violence as a mainstay. In other words, the main purpose of our study is to determine how the religion-based radicalization processes of ISIS in the twenty-first century affect its ideological and organizational membership strategies.