A Lutheran once asked me: “Why was Martin Luther excommunicated? In what way was he heterodox?” Luther was excommunicated in 1521. The radically anti-traditional nature of many (not all) of Luther’s opinions (at least in their early stages) is clearly evident in his three great treatises of 1520: especially in To the Christian Nobility of the German Nation and The Babylonian Captivity of the Church.

I'm not presently trying to defend the rightness or wrongness of Catholic teaching,  but rather, demonstrating that Luther was “heterodox” or “heretical” by the criterion of Catholic teaching. Based directly on statements in these two works, I summarize below how Luther was indisputably heterodox by 1520: judged by existing Catholic theological standards.

1. Separation of justification from sanctification.
2. Extrinsic, forensic, imputed justification.
3. Fiduciary faith.
4. Private judgment over against ecclesial infallibility.
5. Rejection of seven deuterocanonical books.
6. Denial of venial sin.
7. Denial of merit.
8. Sola Scriptura and radically private judgment: “if we are all priests . . . why should we not also have the power to test and judge what is right or wrong in matters of faith?”
9. Denial that the pope has the right to call a council.
10. Only justified men can do good works.
11. Denial of the sacrament of ordination.
12. Denial of exclusively priestly absolution. Anyone in the Christian community can grant absolution.
13. God has not instituted the office of bishop.
14. God has not instituted the office of the papacy.
15. Priests have no special, indelible character.
16. Temporal authorities have power over the Church; even bishops and popes: “The pope should have no authority over the emperor”.
17. Vows of celibacy are wrong and should be abolished.
18. Denial of papal infallibility.
19. Unrighteous priests or popes lose their authority.
20. The keys of the kingdom were not just given to Peter.
21. Private judgment of every individual to determine matters of faith.
22. Denial that the pope has the right to confirm a council.
23. Denial that the Church has the right to demand celibacy of certain callings.
24. God has not instituted the vocation of monk
25. Feast days should be abolished.
26. Fasts should be strictly optional.
27. Canonization of saints is thoroughly corrupt and should stop.
28. Confirmation is not a sacrament.
29. Indulgences should be abolished.
30. Dispensations should be abolished.
31. Philosophy (Aristotle as prime example) is an unsavory, detrimental influence on Christianity.
32. Transubstantiation is “a monstrous idea.”
33. The Church cannot institute sacraments.
34. Denial that the Mass is a good work.
35. Denial that the Mass is a true sacrifice.
36. Denial of the sacramental notion of ex opere operato.
37. Denial that penance is a sacrament.
38. Assertion that the Catholic Church had “completely abolished” the practice of penance.
39. Claim that the Church had abolished faith as an aspect of penance.
40. Denial of apostolic succession.
41. Any layman who can should call a general council.
42. Penitential works are worthless.
43. The seven sacraments lack any biblical proof.
44. Marriage is not a sacrament.
45. Annulments are a senseless concept and the Church has no right to grant them.
46. Whether divorce is allowable is an open question.
47. Divorced persons should be allowed to remarry.
48. Jesus allowed divorce when one partner committed adultery.
49. The priest’s daily office is “vain repetition.”
50. Extreme unction is not a sacrament (the only two sacraments are baptism and the Eucharist).

It is absolutely evident that Luther was heretical and that the Church was under no obligation to even contend with him at the Diet of Worms in 1521, and equally obvious that the Church should demand that he recant, renounce, and cease teaching these things. He refused, because (as he in effect implied, many times) he knew more than the Church. But no Protestant body would have acted any differently, then or now, in the face of 50 (!) rejections of its own stated doctrines.

What was the alternative? We are supposed to believe that the Church should have said, “Luther, you’re right about these fifty issues. You know better than the entire Church, the history of the Church, and all the wisdom of the saints in past ages who have believed these things. So we will bow to your superior, heaven-sent wisdom and change all fifty beliefs or practices, so we can proceed in a godly direction. Thanks so much for informing us of all these errors!”

We are frequently informed that it is self-evident that Luther was a good, obedient Catholic who only wanted to reform the Church, not overturn or leave it, let alone start a new sect.

But no one who reads Luther's three famous treatises of 1520 can doubt that he had already ceased to be an orthodox Catholic. He did not reluctantly become so because he was unfairly kicked out of the Church by men who would not listen to manifest Scripture and reason.

Therefore, the Church was entirely sensible, reasonable, within her rights, logical, self-consistent, and not hypocritical in the slightest, to simply demand that Luther retract his errors at the Diet of Worms in 1521, and to refuse to argue with him (having already tried on several occasions, anyway), because to do so would have granted his presumption that he was in a position to dispute and debate by his own “authority” what had been the accumulated doctrinal and theological wisdom of the Church for almost 1500 years.

The standard “Protestant version” given for the beginning of Luther's new movement is that he only wished to reform the corruptions and excesses in practice, of the Catholic Church (particularly, abuses of indulgences, whoring bishops or priests, etc.). That was certainly an aspect of his motivation and reasoning, but not all of it, by any means, and later in life he even candidly admitted with great disgust that Lutherans were, on the whole, less pious than Catholics.