After the ‘court’ sentenced him to life imprisonment that is when he figured he had nothing to lose and started preaching.
Best to read the article before judging. I think his preaching was in word and deed before being transferred to a receptive mix. "But the initial curiosity the ISIS terrorists had toward their new devoutly Christian cellmate quickly gave way to brutality and beatings.“Then started the physical attacks – fists to my face, kicking my legs with their shoes still on. They used a wooden stick to beat me and experimented with other modes of torture.” Christian leader Petr Jasek was held for 445 days in Sudan, spending a large portion in a cell with ISIS operatives At one point, Jasek’s body was forced into such arduous positions for extended periods that he was not even able to walk for days on end..."
Many "Christians" are not manifest to be so.