Depends on the Mage. If it is against a Southern Circle Mage, a Templar would win easily because a Templar can dispel a mage's spells and Southern mages do not get sufficient martial training. That includes sustains like Combat Magic, Simmering Shield and Shapeshifter forms. Exceptions to this will include Dalish Arcane Warriors and Grey Warden Arcane Warriors.
However, if it is a Templar versus a Tevinter Arcane Warrior or Tevinter Knight Enchanter, the Templar would be in trouble because Tevinter Arcane Warriors would have trained far more extensively in combat so even if a Templar dispels a Tevinter Arcane Warrior's Combat Magic or Simmering Shield, the Tevinter Arcane Warrior is now a Tevinter Mage with Warrior training.