We will later learn via a flashback that the Sentinels are incredibly gullible - all the Prophet had to do was just randomly show up and tell them about the stupid Prophecy and they instantly believed the random stranger and started crying and volunteering to make sure the stupid thing came true. By any means necessary, it would seem. Whether the Prophet told them exactly how to go about it is unknown, but given what an evil piece of shit he is it would make sense if he straight-up instructed them to be underhanded and cruel.