Complex sentence: The Kurus and Panchalas were considered as foremost among the ruling tribes in ancient India, because they followed the Vedic religion in its dogmatic and purest form.
Split into simpler sentences: The Kurus and Panchalas were considered as foremost among the ruling tribes in ancient India, adhering close to the Vedic religion. They were the proponents of the Vedic religion in its dogmatic and purest form.