Christian's death, though not shown on-screen, has been confirmed in a Official Lost podcast (April 20, 2007 Edition). Even further clarification was received in "Access: Granted", when Carlton Cuse stated "In terms of actually physically corporeally in existence... he's dead".
He is, along with Locke and Yemi, one of three previously deceased bodies that later appeared to be living people. What's interesting to note is that all three of these men were already dead, and on airplanes, before they came to the Island. A few of Yemi's appearances were confirmed to be influenced kwa the Monster in the Official Lost Podcast (March 21, 2008 Edition), however, Christian was confirmed as being one of three characters (Yemi and Kate's horse being the other two) to be classified as "undead".So he only acts and help Jacob,as I thiNK