Years ago there was a parishioner at the church I served who was in need of a heart transplant. She was a young woman whose condition was so serious she was admitted to the hospital for care as she waited. She did just that for several weeks when the news came that a heart was now available. It wasn't very long before she was having the surgery that would save her life.
Once the donor heart is removed from the body, it should be transplanted in the recipient within four to five hours. Time is of the essence; and there is a flurry of activity in between the time the heart is transferred from one person to another. The word "immediately" applies well in such a situation.
There is an urgency to the call of God on our lives. It is a matter of life and death. In Mark's Gospel, Jesus is always in a hurry. We are told that immediately Jesus went here and immediately he went there. Immediately Jesus said this and immediately he did that. Mark uses the word "immediately" more than the other three Gospels combined. Mark's Gospel was likely written after the Apostle Paul's death, but I think he would have approved of Mark's portrayal of Jesus' ministry as one of impending consequences.
Why does Paul have such a sense of urgency? Perhaps it has to do with the fact that Christ's work of reconciliation has already started. Richard Hays writes,
...this message of reconciliation is not just a promise of life after death in heaven, not a hope of bliss in a distant disembodied future. Rather, it is a message announcing that God’s work of reconciliation has begun. The ministry of reconciliation has begun, and we are caught up in it. That is extraordinarily good news, because it means that even in the midst of present sufferings, we can trust that God’s reconciling power will prevail. The New Creation is no vain hope; those who are taught to know reality through the cross and resurrection of Jesus know that it is the true destiny of the world -- a world created, sustained and reconciled by the love of Christ.It is incorrect to believe that reconciliation to God and to one another is a future event only-- that work has already started. There is no time to waste.
Moreover, we are not only reconciled; in Christ we are also reconcilers. Paul reminds the fractured and contentious Corinthian Church of their responsibility. It's rather unfortunate that this reading begins at 5:20 and excludes verse 19: "...in Christ God was reconciling the world to himself,* not counting their trespasses against them, and entrusting the message of reconciliation to us."
How can the Corinthians engage as ambassadors of Christ's reconciling ministry when they have not been reconciled themselves to God each other? The fighting and terrible treatment of one another demonstrates that they still need to be reconciled themselves. Paul's entreaty to the Corinthians that they be reconciled to God (5:20) was not for their sake alone; but it was necessary if they were to be faithful ambassadors for Jesus Christ.
There is indeed no time to waste. The acceptable time of salvation is now.