Fellow athiest here and this drawing is pretty bad ass. the way you incorporated the Halo is an idea I never seen come to light when it comes to the damned. The facial structure is great and overall detail is subperb in my opinion
It's a Pentagram. People stand in this circle with the 5-point star (usually drawn on the ground) while they summon dark spirits. The circle is said to serve as a shield to protect the people in it from the powerful evil forces that are summoned. This may even be one of the MANY demons you can contact while in such a ritual (NO JOKE).
Love the jagged muscle work , (the pecs, abs and delts) would love too see a similar version with more of an action pose, ya know see what this guy can really do, see his personality and what not if you know what I mean.