You were raised from a young age to believe one thing to be true. That thing being that your God or gods should be the decider(s) of what magic is and how it should be used. Your main goal is to rid the world of the magic you believe is impure, which in your mind is any that doesn't follow your strict religious code. Do you worship the god/goddess of life? Then any magic that takes away the life of another creature might be a problem for you, and it's your job to remove it from existence. Are you a worshiper of a god that doesn't like living things? Then obviously you hate healing or resurrection spells. They must be removed willingly or by force. Your gods' will is law and you are the one that deals out justice in their name. 

Level 3: Anti-magic smite

You can use your chosen smite as normal, but when you do you gain an additional 1d8+1 to add to the roll. When applied you deny the casher targeted with this smite the use of a spell slot equal to the number rolled. (An 8 on 1d8+1 means that the caster can't use an 8th level spell as they lose that slot) The caster can resist this by making a wisdom save equal to 8+your charisma modifier+the total rolled. 

Level 7: Concentration breaker

When you use anti-magic smite during your attack you can cause the caster's concentration to falter far easier than most can. Upon a successful hit you can cause the caster to make a constitution save as well as the wisdom save. Regardless of the results of the wisdom save the constitution save must be equal to 8+your charisma modifier+the number rolled. On a fail the spell ends. 

Level 15: Anti-magic explosion

You have grown so good at enforcing your gods' will upon others that you can cast your anti-magic smite at will in a burst of energy that stretches 60ft in all directions around you. This will effect all who are hit by it and will remove a spell slot equal to the level rolled by the 1d8+1 you would roll for your anti-magic smite ability. 

Level 20: Divine retribution

You have tried to purge the tainted magic from the sinners' souls with mixed results. You must do something more dramatic to prove your point. You call upon your gods' and they have granted you a favor. Now when you roll your first 1d8+1 that is the percentage of the damage that you will send back at the caster instead of taking that damage yourself. (If you roll an 8 that will be 8% that you avoid, and instead send back to the caster) This will only work on one caster at a time, and can't be moved until that caster is defeated or until you have deemed them as cleansed of their sinful impure magic.