The sin of our first parents is a poison, we are cursed with the knowledge of good and evil. It hunts us from within, it clings to our heart and soul till it drains the last drop of blood. There is no escaping death, it has us where it wants us and it's going to draw us into the grave no matter how hard we fight it. God has the only cure, in God's very nature the righteousness we desperately need is found only in Christ. Jesus took on death, to wrestle us away from death. It was impossible for death to hold him and that is what atonement was inteded for, to release us from the power of death by the power of an indestructable life. Atonement literally means, 'at one moment', that moment was when the high priest entered the holy of holies. He had incence in one hand and the blood of the lamb in the other. One mistake and he would be dragged out by a rope tied to his ankle.
The Jews, converted at Pentecost would have known the gravity of Paul describing to atonement of Christ. Everything hinged on this sacrifice, it covered the sins of an entire nation. It's no coincidence Jesus is called the Lamb in Revelations, it communicates the sacrifice that broke sins hold on the world.
Death could not hold him and sin could not entice him. God could forgive because righteousness can no be provided, by grace through faith, death must yeild. In Christ we enter the holy of holies, where our fath holds like an anchor. Our lives are vain, faulty and fragile. But now the righteousness uthat is by faith has been revealed. Bear in mind the discussion of atement in Romas 3:25 starts in the 21st verse with ''but now'. The context of the verse is informative