People will tell you, ‘Look, there He is!: False messiahs have appeared throughout history. In the first century, many people claimed to be God's agents of salvation (see Acts 5:36–37; 21:38). Jesus warned his disciples not to follow these false leaders.
just as lightning flashes: Lightning lights up the whole sky and is visible to everyone in the area. Similarly, when Jesus suddenly returns, it will be evident to everyone.