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In early Islamic centuries, contrary to the fact that many remained temples from Iranian ancient religions were turned into mosques, most of the mosques were built following the model established by Holy Prophet of Islam in building his first mosque, i.e. a mosque with pillared prayer hall (Khorasaani style). In the 5th century (Saljuqi era) the formal architecture of this sort of mosques began to change into a four-porch mosque, the form of which soon was adopted in making mosques across the land. This article tries to discuss the reasons why that happened and to discuss further that the prevalent ideas and traditions of the time this four-porch mosque first appeared along with the sacred importance of the Cross in Iranian culture may shed light on the reasons this shift came about during that time.