be not a matter of if, but when

To have a particular outcome that is all but certain, though the date or time at which it comes to pass is not. A: "Are you sure we need this expensive antivirus software? I don't really think we're at much risk." B: "Look, when it comes to cyber attacks, it's not a matter of if, but when." I could never understand my parents' need to constantly prepare for the apocalypse. To them, it wasn't a matter of if, but when it would happen. She's a toddler, so tantrums are not a matter of if, but when. I hope you're up for this.