If you can manage to get approx 12 inch x 12 inch x 12 inch cube of decent concrete in the ground, that is not gonna come out easy I guarantee that.
I've had to kango fence posts with less concrete out even though the post is a good lever because it wouldn't budge.
You won't have a problem as long as the potential thieves don't turn up with a digger
Rapid hardening or rapid setting cement is used in the same quantities as regular btw.