Why couldn't johnny tell mr lapham that they were working on Sunday?

Johnny couldn't tell Mr. Lapham that they were working on Sunday because he feared the repercussions of breaking the Sabbath. The Lapham family deeply adhered to their religious beliefs, and acknowledging work on that day would have gone against their principles. Additionally, Johnny was concerned about the potential loss of respect from Mr. Lapham and the implications it would have for his role as an apprentice.