Is Pfeiffer a crypto-Jew? That might explain his madness and apparent diabolical spirit at the cult circus in KY, other than the possibility that the Mexican warlock has some sort of control file on him besides a spell.
When there is a simple explanation, there is no need to go digging for a rare or complicated one.
When my car stops running, and I discover that the car ran out of gas, I'm not going to take my car in to the mechanic for a complete investigation. Mystery solved.
Likewise, we need to look no further than Pablo -- and human nature -- for an explanation of Fr. Pfeiffer's recent behavior. There is no evidence that Fr. Pfeiffer was anything other than an imperfect human being before he met Pablo. But he's been involved with Pablo for many, many years.
Fr. Pfeiffer is human, with a human personality and failings. Isolation will do X to any human being. Also, humans tend to lean on their strengths to make up for their weaknesses.