On the Paladin tree there are a branch for modifying Smite.
Sacrifice makes the skill cost Health instead of mana.
Atonement reduces the cost of the skill (20-100% - 1-5 points).
If you use other skills to reach negative mana, it will prevent you from casting Smite, both when the skill costs Health, and when it’s cost has been reduced by all 5 points, resulting 100% cost reduction.