Questioning Beliefs is the name of a group of quests, which you can gain access to after you either A) complete a respective companion's quest or B) gain high enough friendship/rivalry points with one. To initiate the quest, simply visit the given companion (like Fenris, or as in this example, the elven mage Merrill) in their home, and speak with them. You will have to go through a pretty long dialogue exhange, and in the end will either be able to romance the given companion  or simply gain more friendship or rivalry points.