Answer:
We can see in the letter that Lorenzo de 'Medici advises his son to be loyal to the church, above any other consideration, including the loyalty to the state that the Medici ruled: the Duchy of Florence.
However, Lorenzo de 'Medici has an interest in keeping his son loyal to the church. He knowns that his son's loyalty to the church and the pope can link the Duchy of Florence, and the Medici Family, closer to the ecclesiastical power, which was a great political force at the time.