Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to tell family you are leaving your religion, use this factual base: explain what is changing, when it takes effect, what remains private, and any practical impact on them.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Share the news directly and invite only the questions or support you are ready to receive. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not make one recipient responsible for announcing it to everyone or defending your decision. Plan privacy, housing, medical, employment, and personal safety before disclosing a change that may trigger retaliation. You do not need the other person to agree with a boundary for you to follow it consistently.