What Does "Identity in Christ" Mean? Is it Biblical?

Today's question says, "What exactly does it mean when we say our identity is in Christ? I am trying to teach my kids this concept, but honestly I don't really know how to explain it or demonstrate/illustrate it. What does it really mean and what are the applications? And, most importantly, how do we instill this in our kids so that they are inoculated against the world's efforts to mold their identity to something else like radical gender ideology?"