What is the best ally for a fiend

According to the text, jargon rather than argument is a fiend's best ally in keeping a patient out of the Church.

Jargon, not argument, is your best ally in keeping him from the Church. Don't waste time trying to make him think that materialism is true! Make him think it is strong, or stark, or courageous - that it is the philosophy of the future. That's the sort of thing he cares about.