Theology is the science of the knowledge of God; that is, a science that discusses God. Therefore, in a general sense and in terms of its subject, it encompasses all religions; although in a specific sense, it is limited to its founding origin, namely Christianity. The main issue of theology is human beings and the world insofar as they are creations of the Almighty. 
In the beginning, theology was limited to the discussion of the existence of God; however, it gradually encompassed all truths revealed by the Almighty, as well as divine actions and practical subjects such as worship and spirituality. Over time, it found numerous divisions and classifications, among which the most important are the divisions based on preliminaries and principles, namely the division into natural theology or rational theology or creational theology, and transmitted theology or revelatory theology.
Furthermore, one of the most important branches of theology is practical theology, which deals with the practical aspects of religious life and the study of religious actions and behaviors. It includes practical aspects of religion such as religious education, the study of rituals and ceremonies, and even environmental studies. Applied theology is one of the significant sub-branches of practical theology and effective theology, which seeks to apply theology in individual and social life and aims to explain the religious problems of society and provide solutions to these problems. Consequently, it encompasses various fields and study areas and connects with different aspects of individual and social religious life, as well as various sciences in these fields, including religious sociology, religious psychology, political theology, environmental theology, and the relationship of theology with economics, ethics, health, the meaning of life, education, art, and more.