How do you know He's not already here?
The Bible describes the conditions for his return and what he will do.
For as the lightening comets out of the east, and shineh unto the west; so shall the coming of the Son of Man be. (Matthew 23:27)
Read all of Matthew 24. He warns us about false Christs, and false reports of his coming. Let no man deceive you. When he returns, he will gather dead saints and those who are alive and remain--the first resurrection. (Revelation 20) After he has taken his own, he will immediately pour out his wrath (Revelation 8, 15).
No one will miss the fact that he has come and will be terrified, if they are unsaved.