The claims regarding to the existence of God has been encountered in every period and part in the history of mankind. These claims sometimes have only religious or philosophical, or mystical features. However, we can see these three perceptions interlocked, in harmony with each other, or that a religious claim can be nurtured with philosophical sources or a mystic claim can be nurtured by religious and philosophical arguments. Certainly, we can not make a specific differentiation among these three sides but (...) we will be able to see the differences if we define the factors that build the base of them. The most important point that shows the difference among religious, philosophical and mystic claims is the oneness of God in religion (tevhid=la ilahe illallah), ‘the first commentator’ in philosophy (the infinite power, efficient cause), ‘the communion with God’ in mysticism (initiation, joining= La mevcude illa hu). The issue of the existence and characteristics of God offers various features and these differences in the understanding of God create divergent perceptions of universe, world, existence and human. The more the perception of God changes, the more changes happen in all the systems like dominoes. The issue of the oneness and existence of God is a mental process of humans as it is an understanding, interpreting making implications. If we learn the physical development of human (brain and its functions) and Mental Development Phases presented by Jean Piaget, it will be possible explain the various understanding and interpretation of God. The objective and subjective sides of cognizing the God will come out and the way to make a ‘rational’ explanation will be opened. This paper aims to prove that an individual who has reached the formal operational period can have access to a true understanding of God with ‘rational intuition’. (shrink)