Luther's problem was with the Church's practice of selling indulgences.
An indulgence is a remission of punishment for sins.
However, none can match Christianity for sheer numbers.
There are approximately 41,000 different denominations of Christianity.
So, Luther set about writing 95 complaints with the Church's greedy behavior. On October 3, 1517, Luther nailed his 95 Theses to the door of the Church of Wittenberg and sent copies to the higher authorities of the Catholic Church.
To Luther's eyes, the Church was essentially selling salvation. Luther had read the Bible, something most people of his age could not do, since the only available translations of the Bible were in Latin and Greek.
He witnessed poor peasants giving up their life savings to buy an indulgence for a dead relative, in the hope of saving their loved ones from the tortures of purgatory. Luther saw nothing in the Bible that gave the Church the right to charge people for their salvation.
They were just fighting over who would be at its head.
No one who challenged the supremacy of the Roman Catholic Church was allowed to draw breath for long.