You might be pulling punches as an admin.
The game seems to make you play against your players, unlike most modern day games taking the approach of "you're building a story together".
By that I don't mean actively hindering their progress in playing the game, but when a Sin comes in to play, you have to think this Sin wants the Excorsists gone. Not simply subdued, but removed in whatever flavour the Blasphemy express that in.
Depending on the threat level of the sin, It should have used each of its abilities at least once.
Cat I should get your players used to what a Sin might do, but once they are against Cat II+ the Sins should not be pushovers.
The players should be coming out of fights by a hair's breadth, if at all.
The only way I let players feel at ease against a Sin is if they are fighting sins below their level.
If they are already cleared to fight Cat III but decide to take a breather helping a non-exorcist friend deal with their Cat I issue, yeah, I can see them not having too many issues at that point.