If you consider relativism to be endorsed by Luther, then you don’t know Luther or Lutheranism at all. Find a single Confessional Lutheran (that’s real Lutherans, not the Lutheran-in-name-only modernists) who endorses relativism in the least. Better yet, find a single Lutheran confessional document that endorses relativism. You won’t.
However, if we want to see a relativistic muddying-of-the-Truth, we need only read what modern popes have promulgated about ‘Accidental Christianity,’ or their ‘ecumenical’ words stating rather unambiguously that the Koran and the Bible are “the same,” or the Roman Catholic belief that Islam and Christianity worship the same God. Frankly, after Vatican II acknowledged that all religions hold ‘some ray of the Truth,’ what leg does Rome have to stand on? No, dear friend, Relativism is not a Martin Luther problem. It’s a secular danger that’s been tightening its grip on popular Catholicism for at least a century and a half.
As for splinters and factions, do consider that there was no singular “Reformation.” There was a Lutheran Reformation, a Zwinglian Reformation, an English Reformation, a Roman Counter-Reformation, a French Reformation, and these all interconnected. None of it can be stapled down to one country or one church or one man. History isn’t that clean-cut: Luther was partly to blame; Pope Leo X was partly to blame; Zwingli and, later, Calvin would be partly to blame, Muntzer, the Zwickau Prophets, Henry VIII, Eck, Melanchthon, Cajetan, Erasmus, Charles V, Frederick the Wise, John the Steadfast, Duke Maurice… we couldn’t even pretend to make an exhaustive list.
The fault lies in all Christians. When we acknowledge that, we can begin moving toward a meaningful dialogue led by the Spirit into all Truth. And so, I’d prefer to take the advice of very wise men:
[L]arge communities [have] became separated from full communion with the Catholic Church - for which, often enough, men of both sides were to blame.