Basically, Templars are the jailkeepers and executioners for mages. All mages are feared because they can potentially be possessed by demons (turning them into powerful abominations, which slaughter indiscriminately), or alternatively they can use blood magic, which allows them to control someone's mind, but also has neat tricks like causing people to haemorrhage uncontrollably. In particular, the religion of the setting has strong prohibitions against the use of blood magic because the enemies of the religion's founder used it, and because, according to said religion, darkspawn and the Blight are an indirect result of blood magic. Oh, and it's mostly learnt from demons, since knowledge of blood magic is banned everywhere outside the Tevinter Imperium, which means a blood mage is starting down the long, miserable road to becoming an abomination in most cases.
So, mages are controlled by an organization called the Circle, which is in turn controlled by the Templars, who are the militant arm of the Chantry. They hunt blood mages and those mages who operate outside the Circle, and have the authority to kill mages inside the Circle they suspect of corruption, whether of the "talking to demons" or "blood magic" type.
Cullen, one of the Templars, has a crush on the female mage warden.
In-game events allow the warden to become a blood mage, if one wishes. Wardens are allowed to use any method to stop the blight, including blood magic, and operate outside of the control of the Circle, so for a lot of Templars, they are practically apostates.
A...particular series of in-game events cause Cullen to have considerable mistrust of blood mages, and not unfairly. He kinda has a mental breakdown at the end, if you don't side with him, and he's sent off to listen to whale music or whatever by his superiors until he calms down. He's then reassigned to the most paranoid Templar command for the events of the second game, which has a lot more to do with the tension between mages and templars, and he plays a suitably larger role.
At this point, there is no clear winner on the class/origin poll, so if you wanted to change it, this would be the choice.
