“Why does Paul beseech the Corinthians to be reconciled to God (2 Cor. 5:20) if they were already justified ‘saints’ (1 Cor. 1:2; 6:11)?”
“…be ye reconciled to God” (2 Cor. 5:20).
In this verse, Paul is not telling the Corinthians to be reconciled to God, he is telling them what to tell unbelievers.
If we back up to 2 Cor. 5:18, we see Paul tell the believers in Corinth that God “hath given to us the ministry of reconciliation.” He then goes on to define the ministry of reconciliation when he begins the very next verse with the words “to wit.” Those words mean “namely” or “that is to say.” So Paul is telling believers what to say to unsaved people when they go forth with the ministry of reconciliation. We might paraphrase him to say:
“God has given us the ministry of reconciliation, so go out and tell the lost, ‘God was in Christ as He hung on the cross, imputing your trespasses to Him and not to you. He then committed this message of reconciliation to us believers, and now we are His ambassadors. If He were here, He’d be begging you to be reconciled to Him, but He’s not, we’re here in His stead.’”
The Corinthians were already righteous (1 Cor. 1:30), so we know Paul was telling them to tell unsaved people that they “might” be made righteous (2 Cor. 5:20) by believing the gospel. source