Final answer:
Major trends in religions and belief systems over the millennia include the adoption of monotheism, the development of ethical principles, and the evolution of religious beliefs and practices.
Step-by-step explanation:
In the study of religions and belief systems over the millennia, there are several major trends that have emerged. One trend is the adoption and spread of monotheism, the belief in a single deity such as in Judaism, Christianity, and Islam. Another trend is the exploration and development of ethical principles, such as the pursuit of a 'good' life and moral values. Lastly, there is a trend of adaptation and evolution within religions, where beliefs and practices change slowly over time while retaining older features and adding new ones.