Holy Symbol Expertise is an implement training feat, introduced in the Class Compendium, and was included in Heroes of Shadow supplement. [Dr398:CC:Feats][HoS:157]
Holy symbols are signs of faith. Yours represents a god or a cause that is dear to you. Your resolve is strengthened whenever you smite a foe with your symbol.[HoS:157]
Benefit: You gain a +1 feat bonus to implement attack rolls made with a holy symbol, which increases to +2 and +3 at 11th level and 21st level respectively. When the adventurer attacks an enemy with an implement attack using a holy symbol, the enemies cannot gain combat advantage against the adventurer until the start of his or her next turn, unless using a power or another ability that states the adventurer gains combat advantage.