Baha'u'llah's explanation of "Seal of the Prophets"
"There is no God but One God, and all the Messengers, from the beginning to the Seal of the Prophets (Muhammad) were sent on the part of the True One!" -Abdu'l-Baha, Tablets of the Divine Plan, page 79
" ...how many are those who, through failure to understand its meaning, have allowed the term "Seal of the Prophets" to obscure their understanding, and deprive them of the grace of all His manifold bounties! Hath not Muhammad, Himself, declared: "I am all the Prophets?" Hath He not said as We have already mentioned: "I am Adam, Noah, Moses, and Jesus?" Why should Muhammad, that immortal Beauty, Who hath said: "I am the first Adam" be incapable of saying also: "I am the last Adam"? For even as He regarded Himself to be the "First of the Prophets" - that is Adam - in like manner, the "Seal of the Prophets" is also applicable unto that Divine Beauty. It is admittedly obvious that being the "First of the Prophets," He likewise is their "Seal."
The mystery of this theme hath, in this Dispensation, been a sore test unto all mankind. Behold, how many are those who, clinging unto these words, have disbelieved Him Who is their true Revealer. What, We ask, could this people presume the terms "first" and "last" - when referring to God - glorified be His Name! - to mean? If they maintain that these terms bear reference to this material universe, how could it be possible, when the visible order of things is still manifestly existing? Nay, in this instance, by "first" is meant no other than the "last" and by "last" no other than the "first."
Even as in the "Beginning that hath no beginnings" the term "last" is truly applicable unto Him who is the Educator of the visible and of the invisible, in like manner, are the terms "first" and "last" applicable unto His Manifestations. They are at the same time the Exponents of both the "first" and the "last." Whilst established upon the seat of the "first," they occupy the throne of the "last." Were a discerning eye to be found, it will readily perceive that the exponents of the "first" and the "last," of the "manifest" and the "hidden," of the "beginning" and the "seal" are none other than these holy Beings, these Essences of Detachment, these divine Souls. And wert thou to soar in the holy realm of "God was alone, there was none else besides Him," thou wilt find in that Court all these names utterly non-existent and completely forgotten. Then will thine eyes no longer be obscured by these veils, these terms, and allusions. How ethereal and lofty is this station, unto which even Gabriel, unshepherded, can never attain, and the Bird of Heaven, unassisted, can never reach!" -Baha'u'llah, Kitab-i-Iqan, pages 162-164