The Oath of Redemption paladin has the 20th-level feature Emissary of Redemption (Xanathar's Guide to Everything, p. 39):
Emissary of Redemption
At 20th level, you become an Avatar of Peace, which gives you two benefits:
You have resistance to all damage dealt by other creatures (their attacks, spells, and other effects).
Whenever a creature hits you with an attack, it takes radiant damage equal to half the damage you take from the attack.
It states that you reflect half the damage you take so it seems that he should take 1/4 of the total attack. I'm not aware of any rule about when the resistance applies.
Does Emissary of Redemption reflect damage before or after it applies resistance?
(I'm pretty sure it should be 1/4, just because it would be too strong otherwise, and my master will probably be using it this way too.)