Jacob leads Jack down to a creek as Sawyer, Kate and Hurley watch from a distance. Sawyer cracks a joke saying "And I thought that guy had a God complex before." Kate says "James" in a scolding tone and he says "I know." Hurley says "I'm glad it's just not me."
By the creek, Jacob tells Jack that over the ridge at the bamboo field where he first woke up on the island, that's where the heart of the island is, which he needs to protect from Locke. Jack says that there's nothing past the bamboo, but Jacob says that there is and he'll now be able to find it.
Jacob then asks Jack if he has a cup and Jack gets one from his bag. Jacob reaches into the water and pulls some out, saying in Latin as he does so "Nam non accipimus hoc quasi vulgarem potionem, sed ut ille sit quasi unus mecum," which means "Because we don't accept this as a simple potion, but so that he shall be as one with me." He releases the water back into the creek, puts some into the cup and gives it to Jack to drink. Jack asks how long he will have to do the job, and Jacob says as long as he can. Jack drinks all the water in the cup and then Jacob puts his hand on his shoulder and says "Now you're like me."
The ritual mirrors the passing of power from Mother to Jacob that we saw in "Across the Sea." It's interesting that the light (or the source or heart of the island) is only able to be found by the protector, but it's so close from where Jack's island story began.