I think he means serving other people. In other books he always spoke of the special role of the Minister (big M). So to "minister" probably means to serve others within one's sphere of calling.

Would you not agree that it is the calling of every Christian to serve others in their own spheres and according to their stations in life?

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Pergamum


"If a commission by an earthly king is considered a honor, how can a commission by a Heavenly King be considered a sacrifice?"
-- David Livingstone