Many, many, many books and essays have been written on the differences between Lutherans and Roman Catholics. (In other words, I am not going to tell you anything new. For the most significant differences, you may want to reread the Augsburg Confession and its Apology.)
There are ongoing and serious differences between our two confessions. One question that is often asked is about the relationship between Christians on earth and those Christians who have died and are with Christ in heaven. Lutherans, like Roman Catholics, confess the “communion of saints,” or the union we share with all Christians, living and dead, because we are all joined to the one Jesus. The difference is not in that confession but in the relationship between those who await the resurrection on earth and those who await it in heaven. For Roman Catholics, that communion means that the saints (i.e., those who have died and are free from sin) are able to contribute to a treasury of merit from which those on earth can draw (by, e.g., indulgences). It also means that Christians on this side of death are able to communicate with those on the other side of death, just as you might ask another Christian to pray for you here on earth. (See Catechism of the Catholic Church [CCC], 958.) This is what “prayer” to the saints means for Roman Catholics.
Lutherans acknowledge that those in heaven pray for us, just as they did on earth (Apology of the Augsburg Confession, Article 21, paragraphs 9, 27). However, since we have neither a command to ask them to intercede for us nor a promise that they can hear us, we should not directly invoke them. Neither do they have merit before God that we can access. Jesus’ merit is complete and full, and neither do we need more merit than His nor is there any Christian merit (even for the “holiest” saint) other than what He gives.
We also acknowledge the great grace given to the blessed virgin Mary by God, that she should bear and give birth to the Son of God in flesh. She is, indeed, highly favored, and we revere her because of that divine favor, just as we revere the apostles, whom Jesus chose. We confess that she is properly and rightly called the “mother of God,” because Jesus, even from His conception, was fully God as well as fully man. But we do not believe that Mary was conceived without sin (the “immaculate conception”) or that we should approach her with requests, which she then takes to Jesus. As the Apology puts it, “Although she is most worthy of the most plentiful honors, yet she does not want to be made equal to Christ. Instead, she wants us to consider and follow her example” (Apology, Article 21, paragraph 27).
Another significant difference is the penitential system of the Roman Catholic Church. For Rome, Baptism, Absolution, penance, and purgatory are all tied together. Jesus has taken away the eternal punishment for sin, and Baptism cleanses from original sin, but the temporal consequences of one’s sins remain. So, Rome believes, Confession-Absolution-penance and partaking in the mass are the ways in which one deals with sins after Baptism and the temporal (this-worldly) consequences of our sins. Whatever is not confessed and absolved—and where penance is not performed—must be “purged” from the sinner before he or she enters heaven, hence, purgatory. Only people who are on their way to eternal salvation enter purgatory. (See CCC, 1030–32.)
Lutherans, too, believe that there are temporal consequences to our sins (e.g., a person might go to prison for a crime, even if that person is absolved of his or her sin; we hurt people, and so we must rebuild their trust). That deals with our righteousness before people. But in our righteousness before God, Jesus’ blood and righteousness covers all our sins, both their eternal and temporal consequences. When we die, our sinful flesh will die too; when we are raised from the dead with restored bodies and souls, there will be no more sin and death. We are “purged” from sin by Jesus, since we are joined to His death, as well as to His resurrection, in Holy Baptism. Absolution, too, is a delivering of that forgiveness to repentant sinners, and our forgiveness does not depend on whether we do penance but on what Jesus has done. If we sin against other people, we may indeed have to do things to rebuild or restore relationships, but this work flows from and depends on Jesus’ forgiveness, not the other way around.
The heart of the disagreement between Wittenberg and Rome, from the very beginning and up to this day (the Joint Declaration on the Doctrine of Justification notwithstanding), revolved around how a person can be judged righteous before God. Rome holds that our righteousness before God and our righteousness before people lead in the same direction: toward God. So we are justified by faith and by love, which are combined into one kind of righteousness. Roman Catholics can confess that Christians are saved by grace alone (which initiates and sustains our life in Christ), but because they understand justification as both “the remission of sins” and “the sanctification and renewal of the inner man” (CCC, 1989, citing the Council of Trent), they would deny that we are saved by faith alone.
Lutherans do not deny the necessity of love and good works, but we separate love from justification and place it properly on earth, before other people. In other words, there are two kinds of righteousness: (1) righteousness before God, purely by grace, which is received by faith alone; and (2) our love (good works) for other people. This is also righteousness—outward, which people can see (e.g., Matthew 5:16)—but it does not contribute to our good standing before God (you can’t be more righteous than Jesus!). The question of justification always comes down to where our good works, commanded by God, belong: in heaven before God or on earth before people? Is the direction of our good works toward God in terms of our holiness (even if flowing from and assisted by grace), or is it outward, toward those whom God gives us to serve, and whom He serves through us? Here’s how Gustaf Wingren summed up Luther’s understanding: God does not need our good works, but our neighbor does (See Luther on Vocation [Ballast Press, 1999], 10).
While this brief overview certainly does not exhaust either the similarities or the differences between Lutherans and Roman Catholics, it is always helpful to better understand other traditions and confessions. Better understanding of others helps us see clearly where the differences are (so that we do not argue against straw men), and it also helps us better articulate what we believe. Further, when we work to understand what other Christians confess, we pray more fervently that God would grant unity to His one, holy, catholic (“universal”), apostolic church by His Word and Spirit. Institutions may not be united in this world, but the Spirit continues to grant true understanding and join people to the true confession of His Word. Unity in His Word, and not in a visible institution or leader of a church, is true unity. We do not gain that unity by pretending that differences do not exist but by testing those differences against the Scriptures. Unity is a spiritual and divine work, and God grants it when He gives Christians a common confession of the Scriptures, centered and fulfilled in Christ, the Word of God.
Quotations from Article XXI (IX) “The Invocation of Saints” in the Apology of the Augsburg Confession are from Concordia: The Lutheran Confessions © 2005, 2006 Concordia Publishing House.
Read more from the Augsburg Confessions and the Apology of the Augsburg Confession to understand Luther’s arguments on the differences between Catholicism and Lutheranism.