When Martin Luther nailed his 95th Theses to a church door, he was breaking from the Church that was founded on the apostles. Numerous heretics had emerged before, but, unfortunately, Martin Luther's errors remain.
Now, not all of his 95 points were condemned. However, some of those that were condemned are as follows:
* Baptism does not clean a child's sins
* Fear of death means imperfect charity and purgatory as punishment.
* Penance (contrition, confession, and satisfaction) is untrue, unbiblical.
* Contrition makes a person more of a sinner.
* It is impossible to confess sins.
* A priest cannot absolve your sins unless you believe he has.
* A layperson can, if necessary, absolve your sins.
* A person does not need to show contrition to a priest.
* Faith alone makes one worthy of the Eucharist.
* A person must take both body and blood at communion.
* The Church cannot grant indulgences.
* Excommunications are not something to be feared.
* The pope is not the vicar of Christ.
* The Church has no right to decide upon faith or morals.
* Weakening the Church's councils is desirable.
* Good works are sins.
* You cannot be certain when you are sinning.
* We can have no effect on souls in purgatory, nor know anything about it.
* The Church is greedy.
Theologically the main differences between Luther's thought and the Church's teaching revolved around Sola Fide (Faith Alone). He did not accept the Church's authority and believed that all men could be saved simply through faith. Unfortunately for most people, I think Sola Fide is one of the least understood terms in all of theology.
On the surface it seems like Luther is saying that all you have to do is believe that Christ is God and that he sacrificed himself for the sins of all humanity; if you believe this, then you are saved. When in fact if you analyze his thought carefully you will find that his main difference is not whether good works play a role in your salvation, but whether good works are a result of your salvation. He contends that a saved person will of course carry out good works and that these are the result of grace from God and not our own doing.
So, in a nutshell: Luther believed that a person simply needed to honestly believe in Jesus to be saved, and that all of the other institutions, sacraments, traditions, etc. were unimportant compared to that.
To me, the funny thing about this is that the Catholic Church probably agrees to a certain extent that faith is HUGELY important compared to the rest of it. And, that once you have true faith, following the Church is just a logical next step. I don't think there is a single Catholic out there that believes that he can go to Church each Sunday and go through the motions without honestly loving Jesus and expect to be saved. We don't just love Him, we ADORE Him, we love everything about Him and what He created. And He created this Church so we love our Church too and we follow her teachings.