Religion is vital part of most human societies. Even tough the influence of the church might not be of medievial proportions the clergy often stnds in the front ranks when society is threatened. Evil has to be twarted and it would be a sin to step down when the evil one tries to steal souls from the congregation.
In the world of CoC ole Nick might not be sole representative of Evil. There are other forces that are truely evil that a servant of God needs to put down. If you choose a clerical carreer then this is indeed your job.