It is a contradiction in terms to declare: “I exist to serve myself, and so does everyone else”. Because this statement concedes a distinction between “myself” and all other selves; and since myself MUST be singular/absolute–for there is no way to divide or mitigate the metaphysical root of one’s Self–then it is conceded that the metaphysical frame of reference for all other selves is equally singular/absolute. In which case, it is impossible that all other selves exist to serve “myself” because the absolute/singular metaphysical nature of each Self demands that ALL actions of ALL Selves can only and always be in service to the absolute Self which lay at their root.
So…is it rational to claim that you exist to serve God?
No. It isn’t. Which is why God never claimed it. Your existence is a function of your Self, and so THAT is what you ALWAYS serve. Whether you like it or not.
The real Christian ethic is to serve and promote yourself and allow others the absolute freedom to do the same. Real Christians preach freedom of association, not cosmic slavery.