Soldier of the Pantheon can't be enchanted or equipped by multicolored Auras and Equipment, it can't be blocked by multicolored creatures, it can't be targeted by multicolored spells or abilities from multicolored sources, and all damage dealt to it by multicolored sources is prevented.

Most hybrid spells and permanents, including the ones in the Return to Ravnica block, are multicolored, even if you cast them with one color of mana.