I was raised a Lutheran. I don't understand what the difference between Lutherans and Catholics is.
While it might be easier to describe the similarities between Lutherans and Catholics, there are significant differences. The outward appearance as seen in the conduct of the assemblies of the church are similar in many ways. The doctrine, or system of belief, is different in very specific ways.
One of the most telling differences is that at least some Lutheran fellowships have as a point of doctrine that the Catholic Church, or specifically the Pope, is the Antichrist. Never mind that the Bible never speaks of one antichrist, but many, the Lutheran Church believes that the Pope in Rome (and resultant Catholic doctrine) has so far removed from what the Bible teaches that they deny the truth of Jesus Christ.
Some Lutheran web sites state the main differences by three phrases: Grace alone; Faith alone; and Scripture alone. I will use these three phrases to show the differences.
Grace alone: While both groups appear, to me, to hold the doctrine of original sin (see http://www.minuteswithmessiah.com/question/origsin.html for my refutation of that doctrine), the Lutherans say that one comes to Christ and is justified by God only through God’s grace. Catholics teach that man has some small part in choosing to obey God; Lutherans teach that man cannot choose unless God calls him to do so. Lutherans do not, however, believe in the Calvinist doctrine that God also chooses who will not obey him, although I fail to see how they differ in practical application.
Faith alone: Notwithstanding that the only place in the Bible that the phrase “faith only” appears, it says that we are saved “not by faith only” (James 2:24), nevertheless a central tenet of Lutheran belief is that one is saved by faith only. Although Lutherans believe in the “sacraments” of baptism and the Lord’s Supper (while the Catholics accept more sacraments, such as marriage, last rites, and others), Lutherans reject that these have any direct role in salvation from sin. Catholics believe one cannot be saved without baptism (although they reject the scripture in that they do not baptize by immersion alone). Lutherans believe that baptism is a means of God’s grace, and is commanded and essential. Yet they believe that salvation is by faith only, and not by the administration of baptism. To some of us that seems contradictory. Some of us cannot even determine exactly where Lutherans and Catholics differ on this issue, other than in the words they use.
Scripture alone: Lutherans believe that the Bible as we know it contains the complete revealed will of God, and that there is and will be no further authoritative revelation. (This is interesting since Luther himself rejected the book of James as an “epistle of straw”, and rejected several other books, including Jude and the Revelation.) Catholics, like Pentecostals and Mormons and others, believe that God continues to reveal his specific will to man in addition to what is in the Bible. In the case of Catholics, they believe that the Pope, when speaking “ex cathedra,” speaks in the place of God, even if it differs from what is clearly written in the Bible. Some Lutherans reject the idea that the Bible has no errors in it, while others accept that absolutely.
From an external view, that is from one who is neither Lutheran nor Catholic, some of the differences are purely semantic. As pointed out, what is the difference between faith alone at the point of baptism and faith plus baptism? What is the difference between the elements of the Lord’s Supper becoming literally the body and blood of Christ (Catholic transubstantiation) and that Christ is “in, with, and under” the elements (Lutheran “real presence”)? From this view, the only real distinction between Lutherans and Catholics is over the role of the Pope.
I admit, however, that I am not an expert on either Catholic or Lutheran doctrine. All of this is based on research I have done using official Catholic and Lutheran web sites. I intentionally did not use what others claim that either of these believe, but only what they specifically say in their own behalf.