The penalty of David’s sin had two parts. First, the child conceived in adultery would die. Second, long-term violence would erupt in David’s family. In fact, he lost three of his favorite sons by the sword as a direct result of sin against his constituted authority: Amnon by violating Tamar; Absalom by insurrection against his father; Adonijah for pre-empting God’s choice of Solomon.
The question may be: how can forgiven sin be penalized? It’s the difference between suffering and dying. All people suffer in some way when sinning, though they may not realize it at the time. Any wrongdoing is against God’s righteousness and no one commits it without being penalized by God’s righteousness. It may be only a guilty conscience; it may, as in David’s sin, have long-lasting effects on self and loved ones.
However, suffering because of sin isn’t the same as dying in sin. Jesus warned the Jews, John 8:24, that unbelief in him left them dying in, not merely suffering from, sin.
In many ways Christians can testify that God forgives. And they rejoice in it. They also carry with them, throughout life the consequences, as Paul did his thorn, (perhaps from his rendezvous with Jesus on the Damascus Road; perhaps through an unrecorded event).
The glory of forgiveness, then, is that all believers can rejoice in any penalty accrued from forgiven sin because it binds us closer to God in grace, as Paul discovered in II Corinthians 12:7-10. Fini