We are told that Jesus died for our sins, because otherwise God could not forgive us. That makes no sense to me. If God wanted to forgive, why doesn't he just forgive? Why does he need the death of his son in order to forgive us?
This is, more-or-less, a statement made in regard to Penal Substitution Theory.
Penal Substitution, as the name suggests, is that the Atonement is primarily understood as Jesus vicariously taking upon Himself the penalty of our sin; and from this idea that this was in some way necessary for God to forgive us. You do, however, make a very important point: Why can't God forgive us anyway?
And, of course, we see precisely that God does forgive people in the Bible anyway. In the story of Jonah, Jonah is sent to preach to the people of Nineveh and the people of Nineveh repent and they are forgiven. In Psalm 51 David famously laments of his sin by having Uriah killed in order that he might gratify his lust for Bathsheba, and we see this statement,
"
For you have no delight in sacrifice;
if I were to give a burnt offering, you would not be pleased.
The sacrifice acceptable to God is a broken spirit;
a broken and contrite heart, O God, you will not despise."
So the idea that God is unable to forgive us without having something die, or as though God has some sort of blood obsession, isn't an exactly biblical idea; it's also not--historically--the Christian idea.
Penal Substitution only shows up, as a Theory of the Atonement, in the post-Reformation period, with its genesis primarily found in the writings of Reformed theologians.
Penal Substitution is a variation of an older idea, Satisfaction Theory. Satisfaction Theory shows up in the 11th century in the writings of St. Anselm of Canterbury. Anselm had written his work
Cur Deus Homo (Why God Became Man) in order to explain why the Incarnation was necessary. In it Anselm argues from the perspective of God as Lord, and the Feudalistic context of Anselm can be seen in that God's lordship is perceived as a feudal lord over his fief. God, as Lord, is deserved of the highest honor, and men in their sin have universally offended God's rightful honor and as Lord it is His right to demand satisfaction against His offended honor; and since none of us are capable of paying this honor debt, God the Son becomes a human being that He might, as God's equal and as one of us make right satisfaction on our behalf. Thus Christ's death satisfies the honor debt owed to God.
This Anselmian model was later taken and changed under St. Thomas Aquinas; for Aquinas it is not God's honor, but rather God's justice that is offended. And thus as before, Christ makes satisfaction by His own righteous obedience--even to the point of death on the cross--in order to right the wrongs of humankind.
In neither the Anselmian or Thomist ideas do we find the idea that God needs to punish someone in order to forgive; but rather we see Christ as one who makes satisfaction on our behalf. Penal Substitution takes Satisfaction Theory further by stating that Christ is punished in our stead in order that we might be spared punishment.
All of these, of course, only really get us to about a thousand years ago in the history of the Church, and it is uniquely Western. For the first thousand years of Christianity, and still today in the Eastern Churches, Satisfaction Theory has never really been a popular idea, and Penal Substitution is completely and entirely foreign to the theology of the Christian East.
Instead, the Eastern Churches continue to teach the more ancient ideas of Recapitulation and Ransom.
Recapitulation Theory is usually identified most prominently with St. Irenaeus of Lyons (c. 190 CE). In short, Recapitulation Theory is that Christ fixes what Adam had broken; where Adam had been disobedient and brought death, Christ was obedient and has brought resurrection from the dead. Namely: Christ in assuming human nature has healed the entire human condition in Himself, from birth to death. So He takes upon Himself the entirety of human nature, and redeems it by uniting it to His own divinity, and through His own righteous life sanctifies it, and in dying He shares in our own human mortality, and then by rising liberates the human condition from death and granting immortality and eternal life to the entire human race. Christ, therefore, is the firstfruits of the resurrection from the dead, and we too--because He rose--shall also rise from the dead and share in the eternal life of the world to come. Here, Christ's death is not a punishment from God, it is instead the participation of God in our own mortality in order to rescue us from death, and in fact to rescue the entirety of all creation.
Ransom Theory is not at the exclusion of Recapitulation Theory, but rather is another facet of the ancient Christian view. Namely that God offers His own Son as a ransom payment to the devil, from the idea that human beings by their sin had become captive to the devil. Christ is the ransom, a much bigger prize for the greedy devil to sink his claws upon than the rest of us measly sinners. But the devil doesn't see what's coming: Christ who in death descends into Hades actually destroys Hades and conquers Satan and Death (c.f. the Harrowing of Hell) and rescues the captives of death, and rising from the dead has become the Victor over all the cosmic powers of this fallen world in order that His victory might become the victory of all creation in the resurrection and renewal of all things.
Taken together, Recapitulation and Ransom, are usually in the modern period known as Christus Victor Theory, a term taken from the same-named book by early 20th century Lutheran theologian Gustaf Aulen.
Since the publication of Aulen's work the idea of Christus Victor has become increasingly more well known in the Christian West, and given both Catholic and Protestant Christians a chance to look at the idea of the Atonement through simultaneously fresh and ancient eyes.
Speaking personally, I subscribe, broadly, to Christus Victor Theory; I regard Penal Substitution to be fundamentally untenable because it is difficult to regard God as anything other than a bloodthirsty tyrant through the lens of Penal Substitution. God is primarily angry, and only tolerates human beings if someone suffers and bleeds; this seems to fly firmly against the view of God contained in Scripture and the Creeds: the God who condescends to meet sinners out of love and kindness in order to rescue them and reconcile them to Himself, and who abandons Himself in love for the world.
-CryptoLutheran