I can't source this, as it's pretty standard religious studies info - but at the most basic level, he disagreed with the Catholic policy of the authority of "The Church". I'm not sure you'd find a lot in the script of the liturgies that you can represent this way - his protest was aimed more at what they were doing outside of services - selling indulgences, emergency baptisms, excommunications, etc - basically acting as direct representatives of God.

Catholics believe that the Bible AND the interpretation of scripture by priests (Tradition) are authoritative, while Luther promoted the idea that the Bible/word alone (sola scriptura) was authoritative. In contrast to the Catholic priest, who spoke for God, the Lutheran pastor preaches on the Scripture itself and the worshippers manage their own relationships with God.

Here is a short description of the difference in the role of Catholic vs. Lutheran clergy.

This idea that each person should read the Bible and the newly developed Gutenberg press caused a massive rise in literacy at the time, by the way.

Luther's ideas developed over his lifetime, but the most salient points of disagreement are in the 95 thesis he wrote. At the time of writing this it didn't seem that he had an intention to create a new denomination and his views on sola scriptura and sola fide were not fully developed.

So, the secondary question becomes when did Luther disagree with the church on various points. In many ways Luther became downright bitter and disagreeable with Catholic doctrine as he aged to the point that you might say he disagreed on everything but the trinity and divinity of Christ.

Do you mean you intend to recreate the form and text of an early 16th century Catholic mass for your play or the modern Catholic rite?