Hi Koen,
This is based on the wikipedia page I linked above: Berardo Maggi was both bishop and lord of Brescia (not very common, I think). His most famous achievement was putting an end to the fight between the Guelf and Ghibelline factions in the city, and that is the subject of the scene. The attached image shows the whole center of the scene, which Wikipedia describes as (left to right): Maggi standing and blessing, a clerk sitting at the altar with his hand upon a book and holding a cross, a standing notary or herald reading a scroll, a kneeling soldier. The arch with the sun, star and moon could be inspired by You are not allowed to view links.
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Login to view. that is part of the same church (in this case, I guess Mary would maybe have been replaced by a star, which seems possible, “Ave Maris Stella” etc.). The wikipedia page also connects the Sun and Moon to You are not allowed to view links.
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Login to view., where the Sun stands for the Pope and the Moon for the Emperor: this fits very well with the overall scene.