Whosoever truly loveth me, whosoever is firm in the Covenant, must regard me as the servant of the Threshold of the Blessed Perfection."

"It is ‘Abdu’l-Bahá’s highest aspiration, therefore, to be a true and faithful servant at the Threshold of the Blessed Beauty. Whosoever truly loveth me, whosoever is firm in the Covenant, must regard me as the servant of the Threshold of the Blessed Perfection. Nevertheless, it is unto the Centre of the Covenant that everyone must turn, for he is the Interpreter of the Book, and all the people of Bahá are under his shadow. Should anyone undertake, of his own accord, to interpret the Book of God in a manner contrary to the explicit text of the pen of the Covenant, it is to be rejected, for it would lead to disunity amongst the loved ones of God.

My purpose is to show that the Blessed Beauty hath neither peer nor likeness. He is unique in His essence, and holy and sanctified in His attributes. I am under His shadow and the servant of His Threshold. 

My hope is that following this explicit text which hath proceeded from the pen of the Covenant, no differences whatsoever may remain, and that the American believers, even as the friends in Persia, may all become united in belief, thus rejoicing the heart of ‘Abdu’l-Bahá and leading to the exaltation of the Cause of God in America. Publish this letter and circulate it throughout America. The Glory of Glories rest upon you.
Should a Persian come to those regions from the East—even, supposing the impossible, my son, or a daughter of ‘Abdu’l-Bahá—without having a letter of permission in my handwriting and bearing my signature, it is forbidden to meet such a person or converse with him. For whosoever cometh without permission hath no aim save sedition and the violation of the Covenant."

(Light of the World: Selected Tablets of ‘Abdu’l-Bahá, #46, part 3)
www.bahai.org/r/980398921