Final answer:
The longest-held position in Christianity and Judaism is the recognition of Abraham as a key patriarch. Christianity, the largest religion today, evolved from Judaism and maintains deep connections to its origins through shared beliefs and traditions.
Step-by-step explanation:
The longest-held position in Christianity and Judaism pertains to their shared origins and the figure of Abraham. As two of the world's major monotheistic faiths, Christianity and Judaism view Abraham as a foundational patriarch. Christianity emerged from Judaism approximately 2,000 years ago in Palestine with the teachings of Jesus of Nazareth, emphasizing concepts such as caritas (charity) and the ethical treatment of others. Over the centuries, it evolved and spread, becoming the world's largest religion, yet maintained its connection to Judaic roots through shared traditions, texts, and holy sites. In particular, Jerusalem holds deep significance for both religions, as a sacred place and historical site associated with pivotal moments in their respective narratives.
In summary, the most longstanding shared belief between Christianity and Judaism is the acknowledgment of Abraham's crucial role in their religio-historical identity, which continues to underpin the essential kinship and theological continuity between these faiths.