Johnny Tremain is a novel about a young silversmith apprentice, Johnny Tremain, who first injures his hand and then becomes part of the Revolutionary War. He is arrogant and disdainful of those he believes are inferior to him.
In contrast, the silversmith Johnny is apprenticed to is a godly man who is interested as much in building Johnny's character as he is in refining his skills. Mr. Lapham is a man of great religious devotion, even closing his shop on Sundays so his family--including his apprentices--can attend church services. In contrast, everyone in the family and all the apprentices are willing to secretly break Mr. Lapham's rule in order to make money. It is during this Sunday work session that Johnny gets hurt.
The word "piety" refers primarily to a person's reflection of godliness, as found in the dictionary source cited below:
"reverence for God or devout fulfillment of religious obligation."
I will not write your sentence for you; however, if I had to use "piety" in a sentence, I would write a sentence about Mr. Lapham. He is the most pious person in the novel.