As human beings, change is very difficult for us. We tend to stay in our comfort zones and do things that are familiar to us. In the first millennium, when universal religions spread, humans were the same. Therefore, although political support from powerful figures played an important role in the spread of Mahayana Buddhism during the first millennium, the main factor contributing to the spread of Mahayana Buddhism, especially in China, was the syncretism, the idea of ​​adapting and assimilating one's own system. faith towards others. Similarly, although the socio-economic appeal of converting to Islam played an important role in the spread of Islam in the first millennium, the main factors that contributed to the spread of Islam were the syncretism, the idea of ​​adapting one's faith to another, and Islamization, the process of surrounding oneself with the culture of Islam and slowly adapting to it. One factor in the spread of Islam was socio-economic incentives for conversion. Although the Arab army quickly conquered large territories, conversion to Islam in these areas occurred more slowly. Muslims tolerate diversity and do not want to impose their religion on people. However, the social and economic benefits immediately began to encourage some, under Arab army rule, to convert. Muslims did not have to pay taxes. “It is likely that some non-believers, given the choice between paying taxes or converting to Islam, submitted to Allah.” By converting, one could improve their business and political standing in the community. Arabic was also the language of trade and commerce, so it was beneficial to assimilate into this culture. All of these incitements happened almost immediately after the Arab army conquered the lands. However, by 750 CE, less than ten percent of the population, in the middle of the article, economic incentives played an important role in the spread of Mahayana Buddhism and Islam, syncretism, the idea of ​​adaptation and adaptation. assimilating one's faith with another, played a vital role in the spread of Mahayana Buddhism and Islam. In our modern society, many of our celebrations and gatherings also use the concept of syncretism. Christmas, as celebrated by European tradition, originated from the pagan celebrations of Christmas. Easter, celebrated by European tradition, also incorporates spring fertility rites. Additionally, the Halloween holiday illustrates the syncretism of Christianity and paganism. Throughout history and up to the present day, syncretism has played a major role in the diffusion and development of ideas. It is therefore fair to say that syncretism was the main factor in the spread of universal religions during the first millennium and it still affects us today..