Why would you think everyone must have the same purpose?
Even among the people who think every person's purpose is to serve God, every person has a different way of doing that. This specific way of serving God is their "purpose".
Strikes me as slightly analogous to Aristotle's claim that every person has happiness as a goal. That tells you relatively little about what each person might actually do, since what makes people happy (i.e., mostly realizing their unique individual potential, for Aristotle, "become who you are") differs from person to person.
Even among the people who think every person's purpose is to serve God, every person has a different way of doing that. This specific way of serving God is their "purpose".
Strikes me as slightly analogous to Aristotle's claim that every person has happiness as a goal. That tells you relatively little about what each person might actually do, since what makes people happy (i.e., mostly realizing their unique individual potential, for Aristotle, "become who you are") differs from person to person.