Warriors have several Mesos abilities like Defensive Stance or Sunder Armor that directly increase the amount of threat they have. Paladins have no direct threat generating abilities so instead they rely on their holy damage to increase their threat.
By itself holy damage does not create anymore threat then other damage types like fire or shadow. But at level sixteen you will gain the spell Righteous Fury which increases the amount of threat you create with holy spells by 60%.
If a mage is constantly pounding it with fire balls then it will focus its attention on him because the mage is the biggest danger. Also, if the enemy notices that all of the damage that it is doing is being healed by a priest it will go after her.
Obviously if you want to create a lot of threat you will want to have this spell active and do as much holy damage as you possibly can.
Seals and Judgments: Righteousness - This is your best choice for a seal when tanking. While active each of your melee hits will also deal extra holy damage. This is the most reliable way of dealing holy damage over a fight and accumulating threat.
Judging this seal will blast the enemy with a burst of holy damage and is a great way to start off a fight at the top of the threat list or to try and taunt the enemy back onto you if you lose agro.