What was the biggest argument martin luther had against the church in his 95 theses

In the 95 Theses, Luther argued against the church's sale of indulgences.  Indulgences were papers from the pope that offered people release from penance they owed for their sins. These theses, posted October 31, 1517, created much furor in the church because indulgences were a major source of church income.  Luther argued that if the pope really had power to remove people's penalties for sin, he should do so freely for all.

It would be good for you to know that several weeks before the famous 95 Theses, Luther had writted "A Disputation against Scholastic Theology," a set of 97 theses aimed at the core teachings of the Roman Catholic church's main theological scholars.  This document didn't attract a lot of notice initially, because it was the sort of doctrinal debate university types would have with each other.  But after the controversy got started and Luther was called upon by his brothers in the Augustinian order of monks to defend his theology, the theology he defended was his attack on scholasticism.  At Heidelberg, in 1518, Luther spoke more fully about the central problem's of Rome's theological system.  Then, in 1525, Luther published The Bondage of the Will, which he considered his most important work.  It was a response to Erasmus of Rotterdam who defended the freedom of the human will.  Luther pointed to the Bible and said our wills are enslaved in sin until God takes action to redeem us by his grace.  Notice that this is the same issue that Luther had been raising in his objections to "scholastic theology" in the Roman Catholic system. Throughout his career, Luther's emphasis was on salvation by grace alone, coupled with a rejection of Rome's system that portrayed God giving rewards to the acts we do by our own wills.