Like many human things, Sorcery is probably best when least interfered with celestially. The Ignes are very hands-off, with Gabriel only possibly appearing, and if so, likely only at a required stage of initiation.
The Corporeal Player's Guide recommends that the GM come up with new rituals, and the description of the Ignes suggests that this is a strong component of their tradition. The Focus skills are based off of Precision rather than Will, so presumably other rituals can be as well, meaning that an Ignes Sorcerer doesn't necessarily have to be any more willful than the basic requirements to be a Sorcerer. Such rituals would especially make sense given the Ignes' concentration on purification and cleansing. The Exorcism skills are Will-based, but the Wards are not, so possibly other defenses can be built on Precision.
At
http://earl.of.sandwich.net/In_Nomine/Mystique.txt I list a variant of Sorcery which is based on Perception, as are most of its skills, with one based on Intelligence; however, depending on the GM's permissiveness it might be entirely possible to ditch the introductory paragraphs and simply use the rituals as another Sorcery tradition.