To be fair, these heretics were actually quite cautious.
They didn't believe this strange "compatriot" just because Duncan took out a Sun Amulet, nor did they readily trust his account of what had happened in the sewer sacrificial site; they observed Duncan's words and behavior all along the way and even performed an additional verification after arriving at the gathering to confirm this stranger's identity—given that they were a group of cultists always in hiding, they had done their best.
But all their screening measures treated Duncan as a "normal human being."
Such methods were meaningless to the captain of the Homeloss.