Mastema is one of the Seven, corresponding to Superbia — Pride. Of all the Seven, Mastema is the most theologically ambiguous: not a rebel against the divine, but its instrument. In the Book of Jubilees, he petitions God to allow him to retain one tenth of the spirits of the Nephilim as his servants, to tempt and test humanity. God grants the request. His authority is not stolen but licensed, which makes it considerably harder to argue against.
Within the Book of Thoth Saga, Mastema's domain is the Pride that does not announce itself — the certainty of one's own correctness, the inability to consider that one might be wrong, the conviction that one's judgment supersedes all others'. He does not make people arrogant. He makes them sure. This is the function that coordinates the Seven's operation: the Superbia that coordinates, in Helen Marsden's analysis, the activities of all the others.