First of all, whether or not this is true depends on how stringently you mean “100%”. If we get right down to it we can’t know that we’re not, say, living in some sort of computer simulation; if that’s the case the “real” world could be anything you could imagine with any sort of arbitrary physics and rules. Nothing (with perhaps the exception of math and logic) can be known for sure. But, that makes for a boring argument. So let’s start with the premise that reality exists and is pretty much how we observe it. Not too much to ask, right?
The first thing you’ll realize when you start thinking about this is that to have a non-zero chance of dying you have to live forever. We’re not talking about just living indefinitely long, replacing organs as you go and whatnot, but instead I should be able to name any time in the future and you’ll still be around. Let’s take a look at what that will take.