“The pope does very well when he grants remission to souls in purgatory ... by way of intercession for them." (Martin Luther, 95 Theses, # 26)
The sale of indulgences (never a part of Catholic doctrine) had been condemned by the Sorbonne in 1482 and again in 1518. Many Catholic priests spoke out against these sales, and Luther joined in when Dominican friar John Tetzel (already known and criticized for his abusive sale of indulgences) began this sale where Luther lived.
Indulgences were for earthly reparations after sins had been forgiven. The resulting power of sins has been likened to feathers in a pillow being released in the wind which are impossible to be gathered up. In the same way, actions that are good can also be seen as feathers that blow away in the wind, but have positive results in the world. (An imperfect analogy, but is the reason the Catholic Church condemned the sale of indulgences. Friar John Tetzel, by all reports (although roundly criticized by Luther and others for his sale of Indulgences), was a great speaker. He died in disgrace, but before his death he and Luther met, and Luther noted he did not disagree with everything Tetzel preached. They parted peaceably.