Please understand: The name 'Abdu'l-Baha did not exist during the
lifetime of Baha'u'llah. Nobody, including Baha'u'llah, used that name in
reference to His eldest son and successor. During Baha'u'llah's lifetime,
He referred to His sons as "branches," and 'Abdu'l-Baha was referred to
generally in one of three titles: The Most Mighty Branch; Aqa, or The
Master; and Sirru'llah, or The Mystery of God. In general, He was
referred to as "Aqa," the Master, by Baha'u'llah and all of the Baha'is.
Following the passing of Baha'u'llah, when, in accordance with the
provision in Baha'u'llah's Will and Testament all of the believers turned
to The Most Mighty Branch; like Judas, and like Cain, and like the
brothers of the pure Joseph, Muhammad-Ali, the brother of The Master was
very jealous of His position. This brother claimed that The Master was
claiming the Station of Prophethood, and was claiming equality with their
father, Baha'u'llah. To make clear His own Station, the Master Himself
created and assumed the title, "The Servant of Baha'u'llah,"
(Abdu'l-Baha), which demonstrated His subservience and lowliness before
the Manifestation of God. At His request, the Baha'is began to use this
title, and this practice has continued to the present day. The use of
the titles "The Mystery of God" and "The Most Mighty Branch" to refer to
Abdu'l-Baha have almost ceased among the Baha'is.
Brent Poirier