In this cartoon, Jesus shares the sentiment of "earthly powers may kill the body but not the soul," by telling Peter, "THEY CANNOT KILL YOU IN A WAY THAT MATTERS." This specific phrasing is a reference to a few internet memes (not related to the Bible) and is a hardcore way to express that something might kill you in one way (like destroying your body) but not in a deeper, more important way (like obliterating your very soul). The hardcore nature of the quote implies that the person saying it doesn't mind dying bodily because they're assured they will persist in some other way. This fits Jesus's assurance in Matthew 10:28, though it does little to comfort the apostles in the cartoon, who DO care about dying bodily, because they're just normal men. They're just innocent men.