'Master's' in the Bible

Food of all kinds is meant for the stomach, and the stomach is meant for food, and God will cause both of them to perish. Yet the body does not exist for the purpose of fornication, but for the Master's service, and the Master exists for the body;

If Timothy pays you a visit, see that he is free from fear in his relations with you; for he is engaged in the Master's work just as I am.