John Owen speaks on the revelation of God’s wrath in the death of Jesus, and touches on why it would be impossible for God to “just forgive” people apart from the atonement:
In him God hath manifested the naturalness of this righteousness [his wrath] unto him, in that it was impossible that it should be diverted from sinners without the interposing of a propitiation. Those who lay the necessity of satisfaction merely upon the account of a free act and determination of the will of God, leave, to my apprehension, no just and indispensable foundation for the death of Christ, but lay it upon a supposition of that which might have been otherwise. But plainly, God, in that he spared not his only Son, but made his soul an offering for sin, and would admit of no atonement but in his blood, hath abundantly manifested that it is of necessity to him (his holiness and righteousness requiring it) to render indignation, wrath, tribulation, and anguish unto sin. And the knowledge of this naturalness of vindictive justice, with the necessity of its execution on supposition of sin, is the only true and useful knowledge of it. To look upon it as that which God may exercise or forbear, makes his justice not a property of his nature, but a free act of his will; and a will to punish where one may do otherwise without injustice, is rather ill-will than justice.
– Communion with God, Part II, Chapter III, Digression II