LIGHT OF INTERVENTION'S DESCRIPTION-
MONSTER CARDS CONNOT BE PLAYED FACE-DOWN. MONSTERS SET IN DEFENSE POSTION ARE PLAYED FACE UP ON THE FIELD AND ARE CONSIDERED SUMMONED

If I played LIGHT OF INTERVENTION after my opponent sets, or has any face down monster cards, would they be flipped and effects applied? :confused:

when you activate Light of Intervention, the face-down monsters already on the field will not be flipped face-up

Just to back Xtone up, Light of Intervention does NOT flip monsters face up. It only makes monsters play after it's actvation have to be played face up. It also prevents card effects such as Book of Moon and Tsukeyomi from being activated(or any turn f/d effect).