bathing illustrations and ritual purification

[MarcoP]

I think that the similarity with Christian iconography may be only superficial, and perhaps even intended to make the document "pass" for such a text. Looks like baptism, looks like a cross, looks like cleansing, nothing weird going on here at all, inquisition censors, move along.
I mean, why encipher a document about baptism and cleansing?

I am not terribly sure that a baptism scene in which the officiant is a naked woman would have been looked at with a benevolent eye.

Anyway, I have not written that this illustration represents a baptism because I don't think it does. 
In my opinion it is somehow reminiscent of illustrations of baptism, so it is possible that it represents something similar (such as some other kind of ritual purification, as proposed by David).