It was not as an instructor but as a spy that Brother Fabian was to come.

The whispers abroad--doubtless spread industriously by his vengeful foe--had not been without effect, and men had begun to suspect that his household was tainted with heresy.

Brother Emmanuel was suspected, his sons were probably suspected as being his pupils, and possibly some other members of his household too.

Brother Fabian was to be sent to act as spy, and if bribed (as was most probable) by the Lord of Mortimer, would doubtless find some cause of offence which could be twisted into an accusation of heresy against someone there.
It was difficult for Sir Oliver to see his way all in a moment.

To oppose this scheme or to submit to it appeared alike dangerous.