There Is No Equivalent Exchange
Edward and Alphonse Elric spent years trying to make the math work. The Gospel is the news that the math was never going to work, and that someone else already paid what they couldn't.
The law of equivalent exchange is this: to obtain something, you must give something of equal value in return. Nothing is created from nothing, nothing is lost into nothing, the universe maintains its balance, and the world is essentially fair. It is simple enough for a child to understand, and it takes the entire series to disprove it. Edward and Alphonse Elric build their whole lives on this principle, and when their mother dies, they try to apply it to the one situation it was never designed to handle. It costs them everything they have.
What follows is a long and genuinely moving story about two brothers who believe, with the kind of faith that gets people through impossible things, that if they can just find the right price and meet it, the ledger will close and they can have back what they lost. Ed gives an arm and a leg. Al gives his entire body. They spend years calculating what remains outstanding, hunting for a way to settle the account, convinced that the universe will honor the transaction if they can only bring enough to the table. It is one of the most compelling portraits of human striving in any medium, and the show never lets you forget that it is also a portrait of futility.
The reason Fullmetal Alchemist: Brotherhood lands as hard as it does is that equivalent exchange is not just alchemy. It is the moral logic of almost every human culture that has ever existed, the intuition that wrong must be paid for, that forgiveness must be earned, that the account has to balance before anyone can rest. You hurt someone, so you need to hurt enough on their behalf. You failed, so you have to succeed enough times to make up for it. You owe something to the universe and the universe is keeping track. It feels true in a way that is almost impossible to argue with, which is part of why the show's slow dismantling of it is so affecting.
The Gospel is, among other things, a direct rejection of the whole system. Paul writes in Romans that the free gift of God is eternal life, and the word he uses for gift is the kind that carries no invoice, the kind that requires nothing in return, the kind that is given precisely because the person receiving it had no means to obtain it any other way. Ed and Al cannot pay back what they owe. The price is simply too high for them to meet on their own terms, and the show, to its credit, lets them find this out the hard way rather than explaining it to them. What the ending of Brotherhood gestures toward is that what could not be earned was received as a gift, that the restoration they have been chasing through years of suffering and sacrifice is not something they finally calculate their way to, but something that comes to them freely.
Grace is the theological name for the thing that breaks the law of equivalent exchange. It is the announcement that the debt has already been paid by someone other than you, that the math has been done and settled, and that what is being asked of you now is not more striving but simply the willingness to receive something you could not have bought at any price.