Jacob apparently has the power to restore life - as he seemed to do for Locke following his eight-story dirt dive. Locke is motionless until touched by Jacob, and then gasps and opens his eyes.
Jacob's opponent (let's call him Anti-Jacob or AJ) on the other hand, demonstrated the power to simulate life, such as appearing in form of John Locke.
In addition to Locke, a number of other nominally dead characters have been seen - chief among them Christian Shepard. This begs the question as to the state of Christian. It's possible he is actually alive - his body resurrected by Jacob following the crash of Flight 815. Remember, Christian was not in his casket.
Alternately, Christian may be dead, and we've been seeing another manifestation of AJ all along. A third possibility is that Christian may be dead, but Jacob can simulate him in the same way AJ does Locke.
So, when we see Christian, is it really him, a simulation by Jacob or AJ, or some combination? At present it's difficult to say.
AJ (as Locke) sent Richard to tell the real Locke that he would have to die to bring everyone back to the Island! Now we know why. Once again, poor Locke has been used and deceived, perhaps for the last time.
We've seen Jacob is a fisherman (or as Jesus called his Apostles, fishers of me). He is a weaver, creating an elaborate tapestry with the thread he made himself. Talk about hitting you over the head with symbolism! His tapestry has a line, in Greek, from The Oddessy. It says "May heaven grant you in all things your heart's desire". He lives under an ancient statue, and when Richard Alpert is asked the question "What lies in the shadow of the statue?" he replies in Latin "He who will protect/save us all". And of course, he wears white. Does this mean he is "good"?