Fireball by John Christopher

The book is a science fiction novel that follows the journey of two cousins, Simon and Brad, who are transported to an alternate Earth after encountering a mysterious fireball. In this parallel world, the Roman Empire never fell, and Christianity never rose to prominence, leading to a vastly different societal structure. As the boys navigate this unfamiliar landscape, they encounter various challenges and must adapt to survive while grappling with the implications of their new reality. The story explores themes of cultural divergence, the impact of religion on society, and the resilience required to face unforeseen circumstances.