The aim of this study is to deal with the relationship between the Jewish monotheism and the Christian Trinity. The common ground of these two belief systems is the Second Temple Judaism, and, therefore, they are historically linked with each other. The study deals with this connection by analysing the historical development of the Jewish monotheism in its confrontation to the Christian Trinity and the historical development of the Christology from Jewish mystical concepts to the form of the Greek philosophy. The study attempts to approach the Jewish and Christian religions by considering how the theory of religion differs from the practice of religion. With the aid of semiotic analysis the study attempts to describe the pitfalls which threaten the mystical nature of the Jewish and Christian religions by transforming them into theories of religion. In Jewish-Christian encounter both parts should attempt to dismantle hidden agendas of philosophical understanding of their concepts of God and discuss how they understand them in the mystical way. The mystical understanding may open the way to frank and humble encounter.