The phrase "let in on" means to reveal or share information with someone. Its antonyms include "keep secret," "withhold," "conceal," "censor," and "lock away." These words indicate the opposite of sharing information and suggest that it's best to keep certain things private, hidden, or undisclosed. Keeping secrets can be justified in situations that involve confidential or sensitive information that requires protection from unauthorized disclosure. However, it's important to note that certain secrets can cause harm and that sharing information can be beneficial to build trust, improve relationships, and promote transparency. In some cases, it may be necessary to reveal or share information that could affect others' lives, safety, or well-being.