God's Word of Law and Gospel. The Word goes out and He does not return empty handed. The Spirit of God finding instruments, preachers He sends to deliver the Good News: "This is the body and the blood of Jesus, given and shed for you." God's Spirit opens our eyes and enlightens us so that we may see clear and true that Jesus' cross calls us and all sinners to come witness the focal event of all history. Here at Golgotha God will reconcile the fallen and sinful world to Himself through the flesh of His Word, His Beloved Son, our Savior.
Of course, nobody can understand the aim and goal of Jesus' bloody suffering and death for sin—the "good" of Good Friday—who hasn't properly distinguished God's Word of Law from His Word of Gospel. At Calvary, Jesus nailed to a cross condemns us at the same time that He shows us God's mercy toward all people.
God's Word of Law tells us what we should do, then Jesus crucified shows us that we haven't done nearly enough or, on the other hand, that we've done too much to sabotage salvation. We're a lost cause and God's Word of Law never stops pointing a finger at us, as if to say: "You are the man!"
Then the Good News says, "Not you, but this Man!" Jesus steps in and perfectly fulfills every "jot and tittle" of the Law for us. In fact, all of the Law and prophets hang on Him. No more shoulds and shouldn'ts with Jesus, just the message that "because Jesus... therefore, you." The Gospel tells us what God has done for us.
God's Word of Law demands we pay the penalty for our sinful rebellion against Him. God's Word of Gospel declares that Jesus suffered, died, and descended into hell in our place. On account of Jesus' taking our punishment onto Himself, all God has for us is free gift from His fatherly hand.
At the Cross, God's Word of Law demands a life for the shedding of blood. Jesus says, "I will be the Lamb who sheds His sacrificial blood for the sin of the world." Now the gates of heaven, which God's Word of Law said were closed and locked, are flung open with the Good News that Jesus has triumphed over our sin, our death, and our hell. Now sinners are invited to the heavenly wedding feast of the Bridegroom, not as guests—but as His Bride.
When God's Word of Law and Gospel are confused or mixed together, as if His Word of Law is Good News, or our heavenly Father's promises demand something from us, His Word is altogether lost and the cross is veiled in a cloud of unknowing. Then Jesus' cry of dereliction could mean anything. It could even be heard as a warning to us to "straighten up and fly right" (if we don't want to suffer the same fate).
But when God's Word of Law and Gospel are tuned up, when they're properly distinguished, then Jesus' words rain down on us like thunderbolts: "Father, forgive them, because they don't know what they are doing." And, "It is finished."
Sinners that we are, we don't know what we're doing, especially in relation to God. The good that we would do, we don't do, and the evil that we hate we do without ceasing, as St. Paul wrote. And yet, bursting out of the revelation that we are sinful, dead men walking, just so many of the devil's playthings, comes the Good News of Jesus Christ: "God made Him who had no sin to be sin for us, so that in Him we may become the righteousness of God" (2 Cor. 5:19-21).
This means the penultimate Word of the Cross is God's Word of Law. All that He demanded from us, all that we should have done and left undone, God's wrath and just killing of us sinful men announced to us from the mouth of a preacher, close heaven to us once and for all time. And worst of all, the Law reveals to us that we deserve it!
Then, as we despair, as we cry and weep, as hell opens up its mouth to devour us, God's ultimate Word of Gospel trumpets from Calvary and its notes fill the entire world: "This is what God has done for you, sinners. His Beloved Son, your Savior, Jesus the Christ has done it all for you! For Christ's sake, you are justified, you will receive mercy. Now, receive new life and new hope, because hell's doors have been torn off their hinges and heaven's gates are thrown open to you today and forever."
The Word of the Cross—Christ crucified for you—is God's Word of Law and Gospel. With it, He condemns and justifies, kills and makes alive, drags down to hell and raises up to heaven.
If we give up, ignore, or lose the proper distinction between God's Word of Law and Gospel the Church will put forward a religion of law. It's fellowship will be marked by self-righteousness and calls for behavior modification. Then Christianity is no different than any other religion.
But properly distinguished—the Law as God's penultimate (alien) Word and the Gospel as God's ultimate (proper) Word—the Church, each one of us, is Christ's ambassador, God making His appeal through us. Reconciled to God in Christ through faith we are sent into the world as messengers of God's Word of Law and Gospel: "Repent and believe the good news, sinners... God made Him who had no sin to become sin for you, so that in [Christ Jesus] you may become the righteousness of God."