"That day shall not come" unless certain things
happen in the world. They will be things that
will be signs showing us the rise of the Antichrist.

Paul says, ". . .for that day shall not come, except
there come the falling away first. . .,"
2 Thessalonians 2:3

"the falling away" here means "to stand away
from where you used to stand." That means or is
translated "apostasy". It will be one of the first
signs of the world at the end of the age.