Painting on Glass, St. Christopher and Baby Jesus

What appears to be a very old, reverse painting on glass, with paper and wood panel. Wood panel is slightly rounded, with thin edges, seemingly to precisely fit the glass. Some sort of altarpiece.

From an estate collection of items ranging from the 16th century to mid 20th. This was discovered with items dated well before the 1800s.

Painting depicts St. Christopher with Baby Jesus on his shoulders. St. Christopher's expression is that of almost bewilderment, as he is wondering why the child is so heavy (according to the story). Detail of St. Christopher's body is exquisite, showing his amazingly fit and muscular build.

Obviously some condition issues, with small voids of wear/age. But it appears to be in great condition for its age. Beautiful detail and color through out. Back of painting shows the numerous colors used by the artist. It is an amazing process.

We do not know much, other than recent online research, about this process or this art form, if this is reverse painting on glass. But I imagine this was an icon from a church at some point.

We would love to know more about this. Please feel free to comment or ask questions. It is a difficult item to take photographs of. The pictures included do not show the true beauty of this piece.