“For if we have been united with him in a death like his, we shall certainly be united with him in a resurrection like his.” -Romans 6:5
The word translated united in this verse is the Greek word συμφυτος (symphytos), meaning organically united. The AV (KJV) translates it planted. It could also be translated engrafted. The idea is that since we areengrafted into the death of Christ, we most certainly shall also be engrafted into his resurrection. Our relationship to Christ is more than simply a conformity to his example. It is a spiritually organic union.
So explains Paul again in his letter to the Corinthians. “But in fact Christ has been raised from the dead, the firstfruits of those who have fallen asleep. For as by a man came death, by a man has come also the resurrection of the dead. For as in Adam all die, so also in Christ shall all be made alive. But each in his own order: Christ the firstfruits, then at his coming those who belong to Christ.” -1 Corinthians 15:20–23
Calvin summarizes the union thus: “Hence as the graft has the same life or death in common with the tree into which it is ingrafted, so it is reasonable that we should be partakers of the life no less than of the death of Christ; for if we are ingrafted according to the likeness of Christ’s death, which was not without a resurrection, then our death shall not be without a resurrection.”