Baptism of Fire is an OSR RPG set in the dawn of the Polish monarchy in the early medieval period. It is a complete rulebook and setting. As a setting, it is "medieval authentic," meaning that it is set in our own historical Earth rather than a fantasy world, but this is the world as the people living in the setting imagined and envisioned it. That is to say, religion, magic, and monsters all exist and function as the paradigm of the early medieval world imagined.
The default setting of Baptism of Fire is a society that is transforming from a Dark Age culture to an early Medieval culture. Poland in the early 11th Century was moving from a sedentary tribal system of "barbarians", to the beginnings of a feudal society, united under a monarchy. As part of the process of successfully making this shift, the Piast rulers introduced the unifying force of Christianity to the land.
The process of unifying disparate people into a single nation was predictably met with resistance. But the Piast dukes were clever enough to make good alliances, including with the more powerful Christian kingdoms that had risen up from the ruins of the Roman Empire. Christianity would help cement a common culture for many local tribes, smoothing over countless ancient divisions and rivalries, and legitimizing the crown. But while Christianity had many progressive ideas that appealed to many people, there would nevertheless be a great many who would refuse to give up their age-old gods and heathen traditions without a fight.
All of this was taking place in a setting that was wild and magical. After the fall of Rome, the population of Europe declined dramatically and failed to grow for five centuries. Monsters, fairies, demons, and dark magic ran wild outside of the safety of human cities. The Christianization of Poland was also the taming of that land, purifying it of the dangers of the dark wild and bringing the light of civilization and its accompanying stability and prosperity. In the standard Baptism of Fire campaign, the player characters are agents of that civilizing force.