in all the world: This points to the future mission to the Gentiles (see Matthew 21:43; 28:16–20). Before the end comes, there will be a time, whether short or long, when the good news spreads among the nations.
Jesus said the end will be marked by three signs. First is "the abomination of desolation" (24:15). Second is severe persecution (24:16–28). Finally comes the sign of the Son of Man (24:29–31).