The note reads only:
Locke had earlier tried to convince Jack that he must stay on the Island ("Through the Looking Glass, Part 1") ("There's No Place Like Home, Part 3") and the two had a longstanding disagreement in philosophy with regards to science vs. faith.
The envelope addressed to Jack. ("316")
Jack originally refused to read the note, putting it in Locke's casket after he put Christian's shoes on him while in Simon’s Butcher Shop. However, the note was given back to him on the plane by Rupa Krishnavani after it was found by the airline security when they were screening his cargo. Jack eventually read the note after a conversation with Ben in which Ben asked Jack if he blamed himself for Locke's suicide. After reading the note, the plane encountered turbulence and the plane was engulfed in a flash of white light. Jack then woke up on the Island.
A fragment of the note. ("316")
Shortly after waking up, Jack looked at a fragment of the suicide note in his hand. Only the words "I wish" and Jack's name were written on it. Jack discarded the fragment onto the ground. ("316")
While recovering from surgery in his afterlife, John muttered in his sleep the phrase "I wish you had believed me", a sign of his beginning to remember his old life (apparently writing the note in particular). Later, when Jack said the same thing, John stopped for a moment in surprise before continuing on. ("The Candidate")