In great conflicts, each side claims to act in accordance with God's will. Both, or one, must be wrong.

This phrase connects us with the fundamental value of Epistemological Humility (The awareness that we might be wrong).
In every war, divorce, or political argument, both parties tell themselves a story where they are the hero and the other is the villain. We weaponize morality (or God, or "The Truth") to justify our actions. Lincoln, writing this to himself during the chaos of the Civil War, exposes the logical fallacy of absolute self-righteousness. It is a warning against the arrogance of believing you possess the monopoly on truth.
Here is why this logic is essential for resolving conflicts:
1. The Impossibility of Dual Truths "God cannot be for and against the same thing at the same time."
If two people fight and both claim "Justice" is on their side, logic dictates that at least one of them is hallucinating.
Understanding this breaks the trance of fanaticism. Just because you feel right doesn't mean the universe agrees with you. Your conviction is not evidence of your correctness.
2. The Danger of "The Both Are Wrong" Scenario Lincoln adds a terrifying possibility: "Both... can be wrong."
We usually assume that if the other guy is wrong, I must be right.
But it is entirely possible that both sides are acting out of ego, blindness, or error. In a toxic argument, often neither side represents the "high ground"—both are just dragging each other into the mud.
3. The Antidote to Cruelty When you believe God (or absolute morality) is exclusively on your side, you give yourself permission to do terrible things to your "enemy."
You stop seeing them as human and start seeing them as an obstacle to "The Truth."
Admitting that you could be the one who is wrong forces you to act with caution, mercy, and restraint. It keeps your humanity intact even in the middle of a fight.
The Golden Rule: "My concern is not whether God is on our side; my greatest concern is to be on God's side, for God is always right." — Abraham Lincoln.