All His followers had longed for, all their hopes for a kingdom on earth, had been shattered. But then God, with one mighty act of power, resurrected His Son to life again and later took Him home on high to be with Him. We know that it was with His shed blood that Christ purchased our redemption, our reconciliation, our justification. Paul is simply pointing out that the Resurrection is the positive proof of the sufficiency of the atonement. It is the Father's signed testimony that He is satisfied with our Substitute, our Sacrifice, our High Priest.
Nothing now stands between sinners and forgiveness.
All that was needed has been accomplished since Jesus has come back from the dead. In His person we see the Father's righteous demands satisfied and peace proclaimed between heaven and earth. Pardon, making us right with God, reconciliation are all secured for us in and through our risen Savior. The Resurrection brought the message of reconciliation to humanity. This brings us to the fourth guarantee that Jesus' resurrection brings to us.