This paper aims at explaining the phenomenon of conversion (to the Shin-Cheon-Ji) and re-conversion (from SCJ to the established Protestant Church) through the rational choice theory. In Master’s degree thesis I analyzed why believers of the established Protestant Churches converted to SCJ based on its doctrinal contents. In that work I paid attention mainly to the analysis of attractive parts of SCJ’s religious goods (doctrine and myth) from the point of religious economy. However, I could not discuss the rational choice theory in detail. Therefore in this paper I focused on ‘cost-reward theory’ of R. Stark and W. S. Bainbridge and comparison between SCJ’s and the established Protestant Churches’ cost-reward systems. In my thesis I analyzed testimonies of conversion provided by SCJ, as well as testimonies of re-conversion provided by Christian Heresy Consultation Center. The argument of this paper is based on the rational choice theory belonging to New Paradigm mentioned by R. Stephen Warner. I analyzed the confessions from a ‘cost-benefit’ perspective to find the reasons for devotees‘ conversion and re-conversion. As the result, the following features were verified. Firstly, I focused on the phenomenon of conversion from the established Protestant Church to SCJ and found out that the costs and benefits demanded by the latter were higher than those of the former. In spite of the high cost, Protestant believers converted to SCJ, because they thought they could get a great benefit, such as a guarantee of religious salvation. But why would a devotee of SCJ re-convert and return to the Protestant Church? In order to find it out I paid attention to the re-conversion cases attributed to the assistance of Christian Heresy Consultation Center. The reason could be found in the characteristics of the benefits supplied by SCJ. The rewards of SCJ are characterized by cognitive category, such as right and wrong, or 100% and 0%. However, people, who got a heresy counseling and re-converted to Protestantism, confessed that they were both emotionally and empirically convinced of salvation. In conclusion we could say that both in case of conversion and re-conversion the reward which believers could gain was so-called “assurance of salvation”. But it was found out that the features of the benefits were different from each other: while one was emotional, the other was cognitive.