This article focuses on the analysis of the Avestan texts, which are the primary source of Zoroastrianism. The study compares the Old Avestan texts with the Young Avestan texts. The religious views in the Old Avestan (Gathic) texts are analyzed, showing how the monotheistic principle present in these texts gradually transformed into the polytheistic elements found in the Young Avestan texts over time. The strengthening of polytheistic elements is highlighted. Although Ahura Mazda is depicted as the central deity, the influence of Mithraism, the worship of specific attributes, and other forms of belief on Zoroastrianism are discussed. Additionally, the social, political, and cultural causes behind this process are also considered. This article contributes to understanding the historical development of the Avesta.