It was considered sinful to follow Niccolo Machiavelli's advice because it suggested in his book The Prince that future leaders should rule with reason and make choices based on ideas of humanism over religious beliefs. Unfortunately this was still fairly dangerous territory for men to speak of in his time, seeing as the Medici (a wealthy family who ruled through the church) was still in power. Machiavelli suggested that the qualities of a leader were of a kind sort but, when needed, could be the exact opposite. He was the one to propose many of the ideas on politics and philosophy that numerous world leaders use today. Therefore, although his ideals were very realistic, is seemed to cause a wave of trouble within the church. This is why it was considered sinful to follow Machiavelli's advice.