Thanks, you guys. I agree with Captain that MC wanted exorcistic effects, i.e., Catholic sacramentals.
I also think that Blatty was hinting at Karras’s identification with the divine Victim (the crucified Christ), as well as human victims (the “crucified” human race). Recall his inability to resist the call of others, from the alcoholic (“couldja help an old altar boy”) to whom he gives his last pocket money (unlike the cinematic Karras, who walks away), to the troubled seminarian, to Chris MacNeil, etc.
The Christian message is to take up your cross and follow Jesus, and frequently in the Roman Empire this meant literally taking up a literal cross (or being executed in some other way). Hence Karras on the cross is symbolic both of the crucified Christ, of Christian service in “taking up your cross daily”, and of compassion toward suffering, “crucified” people in general. Certainly he is being obviously identified with the black boy who had been crucified on rowing oars, and with the headless Christ statue – complete with ingot in its eyes – that appears on the cross just prior to the actual crucified Karras’s appearance.
Maybe I’m reading too much into it, this is just my .02 cents.