After the departure of Christ many appeared who were instrumental
in creating factions, schisms and discussions. It became difficult to
know which one was following the right path. One of these disturbers
was Nestorius, a Syrian, who proclaimed that Christ was not a Prophet
of God. This created a division and sect called the Nestorians. The
Catholics declared Jesus Christ to be the Son of God, even pronouncing
Him to be Deity itself. The Protestants announced the doctrine that
Christ embodied two elements: the human and the divine. In brief,
divisions were created in the religion of God, and it was not known
which was pursuing the right pathway because there was no appointed
center to whom Christ referred everyone, no successor whose word was
a gateway to the truth. If Christ had revealed a Covenant with some
soul, commanding all to cling to his word and interpretation as correct,
it would have been evident which belief and statement was valid and
true.

Inasmuch as there was no appointed explainer of the Book of Christ,
everyone made the claim to authority, saying, "This is the true pathway
and others are not."  To ward off such dissensions as these and prevent
any person from creating a division or sect the Blessed Perfection, Bahá'u'lláh,
appointed a central authoritative Personage, declaring Him to be the
expounder of the Book. This implies that the people in general do not
understand the meanings of the Book, but this appointed One does understand.
Therefore, Bahá'u'lláh said, "He is the explainer of My Book and the
Center of My Testament."  In the last verses of the Book instructions are
revealed, declaring that, "After Me," you must turn toward a special
Personage and "whatsoever He says is correct."
-- `Abdu'l-Bahá, The Promulgation of Universal Peace, p. 381

In the passage above it is indicated Christ did not appoint a successor,
"a gateway of truth" to expound his teachings, whom Christians were to
follow. Based on the Bible and the passages below from other writings of
Abdu'l-Baha, Peter and the rest of the 12 Apostles were Christ's divinely
inspired successors who continued his teachings. A number of prophecies
about the coming of Muhammad and the 12 Imams, The Bab, and Baha'u'llah
came from the Apostles.

In each cycle the guardians and holy souls have been twelve. So Jacob
had twelve sons; in the time of Moses there were twelve heads or chiefs
of the tribes; in the time of Christ there were twelve Apostles; and in
the time of Muhammad there were twelve Imáms.
-- `Abdu'l-Bahá, Some Answered Questions, p. 57.

These twelve gates surround the entire world, that is they are a shelter
for all creatures. And further, these twelve gates are the foundation of
the City of God, the heavenly Jerusalem, and on each one of these
foundations is written the name of one of the Apostles of Christ. That is
to say, each one maketh manifest the perfections, the joyous message, and
the excellency of that holy Being.
-- Selections from the Writings of `Abdu'l-Bahá p. 166.

1:1 The former treatise have I made, O Theophilus, of all that Jesus began
     both to do and teach,

1:2 Until the day in which he was taken up, after that he through the Holy
     Ghost had given commandments unto the apostles whom he had chosen:
-- Bible: Acts.

6:13 And when it was day, he called unto him his disciples: and of them he
      chose twelve, whom also he named apostles;

6:14 Simon, (whom he also named Peter,) and Andrew his brother, James and
      John, Philip and Bartholomew,

6:15 Matthew and Thomas, James the son of Alphaeus, and Simon called
      Zelotes,

6:16 And Judas the brother of James, and Judas Iscariot, which also was
      the traitor.

6:17 And he came down with them, and stood in the plain, and the company
      of his disciples, and a great multitude of people out of all Judaea
      and Jerusalem, and from the sea coast of Tyre and Sidon, which came
      to hear him, and to be healed of their diseases;
-- Bible: Luke,

16:18 And I say also unto thee, That thou art Peter, and upon this rock I
       will build my church; and the gates of hell shall not prevail against it.

16:19 And I will give unto thee the keys of the kingdom of heaven: and
       whatsoever thou shalt bind on earth shall be bound in heaven: and
       whatsoever thou shalt loose on earth shall be loosed in heaven.
-- Bible: Matthew,
 

The Word which the Son concealed is made manifest. It hath been sent down
in the form of the human temple in this day. Blessed be the Lord Who is
the Father! He, verily, is come unto the nations in His most great
majesty. Turn your faces towards Him, O concourse of the righteous...
This is the day whereon the Rock (Peter) crieth out and shouteth, and
celebrateth the praise of its Lord, the All-Possessing, the Most High,
saying: `Lo! The Father is come, and that which ye were promised in the
Kingdom is fulfilled!...'
-- Bahá'u'lláh, Proclamation of Baha'u'llah, p. 84-85
 

2:13 But now in Christ Jesus ye who sometimes were far off are made nigh by
      the blood of Christ...

2:18 For through him we both have access by one Spirit unto the Father.

2:19 Now therefore ye are no more strangers and foreigners, but
      fellow citizens with the saints, and of the household of God;

2:20 And are built upon the foundation of the apostles and prophets, Jesus
      Christ himself being the chief corner stone;
-- Bible: Ephesians

More references on the 12 Apostles.

Emmanuel was indeed the Herald of the Second Coming of Christ, and a
Summoner to the pathway of the Kingdom. It is evident that the Letter is
a member of the Word, and this membership in the Word signifieth that the
Letter is dependent for its value on the Word, that is, it deriveth its
grace from the Word; it has a spiritual kinship with the Word, and is
accounted an integral part of the Word. The Apostles were even as Letters,
and Christ was the essence of the Word Itself; and the meaning of the Word,
which is grace everlasting, cast a splendour on those Letters. Again, since
the Letter is a member of the Word, it therefore, in its inner meaning, is
consonant with the Word.
-- Selections from the Writings of Abdu'l-Baha, p. 60
 

Consider! The station and the confirmation of the apostles in the time of
Christ was not known, and no one looked on them with the feeling of
importance -- nay, rather, they persecuted and ridiculed them. Later on it
became evident what crowns studded with the brilliant jewels of guidance
were placed on the heads of the apostles, Mary Magdalene and Mary the
mother of John.
-- `Abdu'l-Bahá, Tablets of the Divine Plan, p. 39
 

Thou didst ask as to the tenth to the seventeenth verses of the twenty-first
chapter of Saint John the Divine's Revelation. Know thou that according to
mathematical principles, the firmament of this earth's brilliant day-star
hath been divided among twelve constellations, which they call the twelve
zodiacal signs. In the same way, the Sun of Truth shineth out from and
sheddeth its bounties through twelve stations of holiness, and by these
heavenly signs are meant those stainless and unsullied personages who are
the very well-springs of sanctity, and the dawning-points proclaiming the
oneness of God.

Consider how in the days of the Interlocutor (Moses), there were twelve holy
beings who were leaders of the twelve tribes; and likewise in the
dispensation of the Spirit (Christ), note that there were twelve Apostles
gathered within the sheltering shade of that supernal Light, and from those
splendid dawning-points the Sun of Truth shone forth even as the sun in the
sky. Again, in the days of Muhammad, observe that there were twelve
dawning-points of holiness, the manifestors of God's confirming help. Such
is the way of it.

Accordingly did Saint John the Divine tell of twelve gates in his vision, and
twelve foundations. By `that great city, the holy Jerusalem, descending
out of heaven from God' is meant the holy Law of God, and this is set forth
in many Tablets and still to be read in the Scriptures of the Prophets of
the past: for instance, that Jerusalem was seen going out into the
wilderness.

The meaning of the passage is that this heavenly Jerusalem hath twelve gates,
through which the blessed enter into the City of God. These gates are souls
who are as guiding stars, as portals of knowledge and grace; and within
these gates there stand twelve angels. By `angel' is meant the power of the
confirmations of God--that the candle of God's confirming power shineth out
from the lamp-niche of those souls--meaning that every one of those beings
will be granted the most vehement confirming support.

These twelve gates surround the entire world, that is they are a shelter for
all creatures. And further, these twelve gates are the foundation of the
City of God, the heavenly Jerusalem, and on each one of these foundations is
written the name of one of the Apostles of Christ. That is to say, each one
maketh manifest the perfections, the joyous message, and the excellency of
that holy Being.
-- `Abdu'l-Bahá, Selections from the Writings of Abdu'l-Baha, p. 165