What was the message of the ghost of Marley and why was Scrooge being warned?

The message of the ghost of Marley was to warn Scrooge about his selfish and greedy ways, and the consequences he would face if he didn't change his behavior. Scrooge was being warned because he was on a path of isolation and misery that would ultimately lead to his own downfall if he didn't embrace kindness and compassion.