whenever credence is under great stress/pressure he starts sucking hia thumb for comfort
of course, it always happens in secret (credence is quite ashamed of it)
however, as he again fears punishment from someone he trusts/loves he goes into hiding and does what he always does -- now, this time he is caught and his s.o. (it actually doesn't have to be a romantic relationship at all, it can be whatever the author wants it to be with whomever) thinks it to be quite ... Enticing

lots of praise and gentle nudging credence into the right direction happens, eventually they have soft tender sex and credence, it being his first time, gets so overwhelmed he (you might've guessed it) starts sucking his thumb again