“But if a wicked person turns away from all the sins they have committed and keeps all my decrees and does what is just and right, that person will surely live; they will not die. 22 None of the offenses they have committed will be remembered against them. Because of the righteous things they have done, they will live. 23 Do I take any pleasure in the death of the wicked? declares the Sovereign Lord. Rather, am I not pleased when they turn from their ways and live?” (Ezekiel 18:21-23)
But hasn't God already forgiven us? Do we really need to repent? Or does he forgive us "if" we repent? Can it be true that we are forgiven but we also need to be forgiven? If forgiveness can have two different meanings, yes, it can be true. It just so happens that there are two types of forgiveness from God.
One type of forgiveness is unconditional, and the other is conditional. The unconditional kind of forgiveness simply means that he loves us unconditionally and is always doing his very best for us (see here). The other is conditional; because God will not approve of us doing things which destroy us and others. He loves us too much to ignore those things. (If you don't know that you are unconditionally loved by God watch this video.)
So even though we are forgiven, when we start to trust and obey God, he forgives us. Love keeps no record of wrongs. So how does a person trust and obey God today? They trust and obey him by trusting and obeying the Messiah whom he has sent.
Whether in the Old Covenant or the New, God has always required people to trust and obey him in order to forgive. (There have never been sacrifices for serious sins. For an in-depth look at this see Lamb of the Free: Recovering the Varied Sacrificial Understandings of Jesus’s Death by Andrew Remington Rillera.) God did not forgive David because he offered a sacrifice. David offered a sacrifice because he was forgiven.
God requires repentance, not punishment. He wants to save us from our sins. And he cannot start saving us from our sins until we begin to trust and obey him. (In the New Covenant that means trusting and obeying the Messiah. See What is the Gospel? See also Atonement, Justice, and Peace: The Message of the Cross and the Mission of the Church by Darrin W. Snyder Belousek, and Beyond Justification: Liberating Paul’s Gospel by Douglas Campbell.)