Before my father married my mother and conceived me, he was in a serious automobile accident. Had my father died at the age of sixteen, I would have died also, because I was still in my father. His death would have been my death; and not my death only, but also the death of my children, and their children, all the way down the line. We would have all died together with him, having never even lived. But his life, his survival, his overcoming the accident was just as much my life, my survival, and my overcoming.
Spiritually speaking, this is similar to what God has done for us by placing us in Christ. Christ was crucified, dead, buried, resurrected, ascended, and seated; and since we were in Him when it all happened, God considers it to have happened to us as well. We who are His children now share in the very life of Christ, and each of us bear witness to the resurrection and overcoming of Christ. When God raised Him from the dead, He raised us from the dead as well.