Mirror Image description says
Each time a creature targets you with an attack during the spell's duration, roll a d20 to determine whether the attack instead targets one of your duplicates.
The Sacred Flame cantrip does not require the caster to make a spell attack, its effect depends on the target's saving throw instead:
Flame-like radiance descends on a creature that you can see within range. The target must succeed on a Dexterity saving throw or take 1d8 radiant damage.
This implies Sacred Flame automatically targets the creature, revealing the original.
Is it correct, if somebody casts Sacred Flame, the "each time a creature targets you with an attack during the spell's duration" part doesn't work anymore, since everybody already knows where the original is?
Best Answer
No, sacred flame doesn't negate mirror image.
Although yes, (if your DM rules that it doesn't require accurately choosing your target) sacred flame will reveal the target briefly, how mirror image works means that information is immediately obsolete regardless, even for yourself:
It's not possible to keep track of the original. As soon as the sacred flame is done revealing the original by hitting it, mirror image will make sure that everyone who saw which was hit loses track of which one is real. That's what mirror image is for.