The world is but one country and mankind its citizens.

This statement is the plagiarized version of a line of poetry from the 13th century Persian Poet called Saadi:

Human beings are members of a whole,
In creation of one essence and soul.

Baha’u’llah the founder of Baha’ism simply repeated this statement using other words and attributed it to himself.

The statement is simply “stating the obvious”, yes the world is like a big country and yes we are its citizens.