If you want their help then travel to the Circle of the Magi tower, and clear it of the abominations so that the tower can be declared safe. Then you can ask for their aid against the Blight.
As soon as you continue through the double doors into the dorm rooms of the first floor you are locked in. The Templars will refuse to open them again until you have cleared the tower out. You need to reach the very top of the tower where Irving (head of the circle) and the other mages are being held captive. You can chose to side with the mages and gain their aid, or side with the templars and slaughter the mages, gaining their support.