"Word of man" here is a synecdoche for "a message that comes simply from a man." Alternate translation: "not a message that is made up by a man" (See: [[rc://en/ta/man/jit/figs-synecdoche]])
Paul speaks of God's gospel message as if it were a person who was doing work. Alternate translation: "which those of you who believe are listening to and beginning to obey" (See: [[rc://en/ta/man/jit/figs-personification]] and [[rc://en/ta/man/jit/figs-metonymy]])