The meaning of the passage: “.. the Comforter, which is the Holy Ghost..”

“But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.” (St. John 14:26)

The passage in the Gospel of St. John 14:26, is a reference to the Revelation of Bahá’u’lláh, through Whose coming this prophecy was fulfilled. 
- Shoghi Effendi  (From a letter written on behalf of Shoghi Effendi to an individual believer, April 21, 1939; compilation: 'Lights of Guidance')

We are called upon to “follow” the “footsteps” of the disciples of Christ

It behoveth them not to rest for a moment, neither to seek repose. They must disperse themselves in every land, pass by every clime, and travel throughout all regions. Bestirred, without rest, and steadfast to the end, they must raise in every land the triumphal cry, Ya Baha'u'l-Abha! (O Thou the Glory of Glories).... The disciples of Christ forgot themselves and all earthly things, forsook all their cares and belongings, purged themselves of self and passion, and with absolute detachment scattered far and wide and engaged in calling the peoples of the world to the divine guidance; till at last they made the world another world, illumined the surface of the earth, and even to their last hour proved self-sacrificing in the pathway of that beloved one of God. Finally in various lands they suffered glorious martyrdom. Let them that are men of action follow in their footsteps! 
- ‘Abdu’l-Baha  (Quoted by Shoghi Effendi, The Unfolding Destiny of the British Baha'i Community)

"fishers of men” and now: "quickeners of mankind”

The Revelation associated with the Faith of Jesus Christ focused attention primarily on the redemption of the individual and the molding of his conduct, and stressed, as its central theme, the necessity of inculcating a high standard of morality and discipline into man, as the fundamental unit in human society. Nowhere in the Gospels do we find any reference to the unity of nations or the unification of mankind as a whole. When Jesus spoke to those around Him, He addressed them primarily as individuals rather than as component parts of one universal, indivisible entity. The whole surface of the earth was as yet unexplored, and the organization of all its peoples and nations as one unit could, consequently, not be envisaged, how much less proclaimed or established. What other interpretation can be given to these words, addressed specifically by Bahá'u'lláh to the followers of the Gospel, in which the fundamental distinction between the Mission of Jesus Christ, concerning primarily the individual, and His own Message, directed more particularly to mankind as a whole, has been definitely established: "Verily, He [Jesus] said: 'Come ye after Me, and I will make you to become fishers of men.' In this day, however, We say: 'Come ye after Me, that We may make you to become the quickeners of mankind.'" 
- Shoghi Effendi  ('The Promised Day is Come')

The amazing spiritual transformation of Peter

"Peter," 'Abdu'l-Bahá has testified, "according to the history of the Church, was also incapable of keeping count of the days of the week. Whenever he decided to go fishing, he would tie up his weekly food into seven parcels, and every day he would eat one of them, and when he had reached the seventh, he would know that the Sabbath had arrived, and thereupon would observe it." If the Son of Man was capable of infusing into apparently so crude and helpless an instrument such potency as to cause, in the words of Bahá'u'lláh, "the mysteries of wisdom and of utterance to flow out of his mouth," and to exalt him above the rest of His disciples, and render him fit to become His successor and the founder of His Church, how much more can the Father, Who is Bahá'u'lláh, empower the most puny and insignificant among His followers to achieve, for the execution of His purpose, such wonders as would dwarf the mightiest achievements of even the first apostle of Jesus Christ! 
- Shoghi Effendi  ('The Advent of Divine Justice')

The meaning of the Messiah's words “Verily the Son is in the Father, and the Father is in the Son.'[John 14:10]

Know thou that the Divine Essence, which is called the Invisible of the Invisibles, never to be described, beyond the reach of mind -- is sanctified above any mention, any definition or hint or allusion, any acclamation or praise. In the sense that It is that It is, the intellect can never grasp It, and the soul seeking knowledge of It is but a wanderer in the desert, and far astray. 'No vision taketh in Him, but He taketh in all vision: He is the Subtile, the All-Informed.'[1]

When, however, thou dost contemplate the innermost essence of all things, and the individuality of each, thou wilt behold the signs of thy Lord's mercy in every created thing, and see the spreading rays of His Names and Attributes throughout all the realm of being, with evidences which none will deny save the froward and the unaware. Then wilt thou observe that the universe is a scroll that discloseth His hidden secrets, which are preserved in the well-guarded Tablet. And not an atom of all the atoms in existence, not a creature from amongst the creatures but speaketh His praise and telleth of His attributes and names, revealeth the glory of His might and guideth to His oneness and His mercy: and none will gainsay this who hath ears to hear, eyes to see, and a mind that is sound.

Prayer Christ said while being hanged on the cross

Remind them of these words and say unto them: 'Verily did the Pharisees rise up against Messiah, despite the bright beauty of His face and all His comeliness, and they cried out that He was not Messiah [Masih] but a monster [Masikh], because He had claimed to be Almighty God, the sovereign Lord of all, and told them, 'I am God's Son, and verily in the inmost being of His only Son, His mighty Ward, clearly revealed with all His attributes, all His perfections, standeth the Father.' This, they said, was open blasphemy and slander against the Lord according to the clear and irrefutable texts of the Old Testament. Therefore they passed the sentence upon Him, decreeing that His blood be shed, and they hanged Him on the cross, where He cried out, 'O My beloved Lord, how long wilt Thou abandon Me to them? Lift Me up unto Thee, shelter Me close to Thee, make Me a dwelling by Thy throne of glory. Verily art Thou the Answerer of prayers, and Thou art the Clement, the Merciful. O My Lord! Verily this world with all its vastness can no longer contain Me, and I love this cross, out of love for Thy beauty, and yearning for Thy realm on high, and because of this fire, fanned by the gusts of Thy holiness, aflame within My heart. Help me, O Lord, to ascend unto Thee, sustain Me that I may reach unto Thy sacred Threshold, O My loving Lord! Verily Thou art the Merciful, the Possessor of great bounty! Verily Thou art the Generous! Verily Thou art the Compassionate! Verily Thou art the All-Knowing! There is none other God save Thee, the Mighty, the Powerful!' 
- ‘Abdu'l-Baha  ('Selections from the Writings of ‘Abdu'l-Baha')

"O would that all were truly Christian!"

Thou didst begin thy letter with a blessed phrase, saying: 'I am a Christian.' O would that all were truly Christian! It is easy to be a Christian on the tongue, but hard to be a true one. Today some five hundred million souls are Christian, but the real Christian is very rare: he is that soul from whose comely face there shineth the splendour of Christ, and who showeth forth the perfections of the Kingdom; this is a matter of great moment, for to be a Christian is to embody every excellence there is. I hope that thou, too, shalt become a true Christian. Praise thou God that at last, through the divine teachings, thou hast obtained both sight and insight to the highest degree, and hast become firmly rooted in certitude and faith. It is my hope that others as well will achieve illumined eyes and hearing ears, and attain to everlasting life: that these many rivers, each flowing along in diverse and separated beds, will find their way back to the circumambient sea, and merge together and rise up in a single wave of surging oneness; that the unity of truth, through the power of God, will make these illusory differences to vanish away. This is the one essential: for if unity be gained, all other problems will disappear of themselves. 
- ‘Abdu'l-Baha  ('Selections from the Writings of ‘Abdu'l-Baha')