Complex sentence: They advance Luther's positions against what he saw as abusive practices by preachers selling plenary indulgences, which were certificates which would reduce the temporal punishment for sins committed by the purchaser or their loved ones in purgatory.
Split into simpler sentences: They advance Luther's positions against what he saw as abusive practices by preachers selling plenary indulgences. These were certificates which would reduce the temporal punishment for sins committed by the purchaser or their loved ones in purgatory.