From the Writings and Utterances of `Abdu’l-Baha:
This Holy Spirit is the mediator between God and His creatures. It is like a mirror facing the sun. As the pure mirror receives light from the sun and transmits this bounty to others, so the Holy Spirit is the mediator of the Holy Light from the Sun of Reality, which it gives to the sanctified realities. It is adorned with all the divine perfections. Every time it appears, the world is renewed, and a new cycle is founded. The body of the world of humanity puts on a new garment. It can be compared to the spring; whenever it comes, the world passes from one condition to another. Through the advent of the season of spring the black earth and the fields and wildernesses will become verdant and blooming, and all sorts of flowers and sweet-scented herbs will grow; the trees will have new life, and new fruits will appear, and a new cycle is founded. The appearance of the Holy Spirit is like this. Whenever it appears, it renews the world of humanity and gives a new spirit to the human realities: it arrays the world of existence in a praiseworthy garment, dispels the darkness of ignorance, and causes the radiation of the light of perfections.
(`Abdu’l-Baha: Some Answered Questions, p. 145)

Likewise the Holy Spirit is the very cause of the life of man; without the Holy Spirit he would have no intellect, he would be unable to acquire his scientific knowledge by which his great influence over the rest of creation is gained. The illumination of the Holy Spirit gives to man the power of thought, and enables him to make discoveries by which he bends the laws of nature to his will.
The Holy Spirit it is which, through the mediation of the Prophets of God, teaches spiritual virtues to man and enables him to attain Eternal Life.
All these blessings are brought to man by the Holy Spirit; therefore we can understand that the Holy Spirit is the Intermediary between the Creator and the created. The light and heat of the sun cause the earth to be fruitful, and create life in all things that grow; and the Holy Spirit quickens the souls of men.
(`Abdu’l-Baha: Paris Talks, p. 59)

It is only by the breath of the Holy Spirit that spiritual development can come about. No matter how the material world may progress, no matter how splendidly it may adorn itself, it can never be anything but a lifeless body unless the soul is within, for it is the soul that animates the body; the body alone has no real significance. Deprived of the blessings of the Holy Spirit the material body would be inert.
(`Abdu’l-Baha: Paris Talks, p. 133)

Just as the human spirit of life is the cause of coordination among the various parts of the human organism, the Holy Spirit is the controlling cause of the unity and coordination of mankind. That is to say, the bond or oneness of humanity cannot be effectively established save through the power of the Holy Spirit, for the world of humanity is a composite body, and the Holy Spirit is the animating principle of its life.
Therefore, we must strive in order that the power of the Holy Spirit may become effective throughout the world of mankind, that it may confer a new quickening life upon the body politic of the nations and peoples and that all may be guided to the protection and shelter of the Word of God. Then this human world will become angelic, earthly darkness pass away and celestial illumination flood the horizons, human defects be effaced and divine virtues become resplendent. This is possible and real, but only through the power of the Holy Spirit. Today the greatest need of the world is the animating, unifying presence of the Holy Spirit. Until it becomes effective, penetrating and interpenetrating hearts and spirits, and until perfect, reasoning faith shall be implanted in the minds of men, it will be impossible for the social body to be inspired with security and confidence.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 321)

Therefore, the Lord of mankind has caused His holy, divine Manifestations to come into the world. He has revealed His heavenly Books in order to establish spiritual brotherhood and through the power of the Holy Spirit has made it practicable for perfect fraternity to be realized among mankind. And when through the breaths of the Holy Spirit this perfect fraternity and agreement are established amongst men – this brotherhood and love being spiritual in character, this loving-kindness being heavenly, these constraining bonds being divine – a unity appears which is indissoluble, unchanging and never subject to transformation.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 391)

The greatest power of the Holy Spirit exists in the Divine Manifestations of the Truth. Through the power of the Spirit the Heavenly Teaching has been brought into the World of Humanity. Through the power of the Spirit life everlasting has come to the children of men. Through the power of the Spirit the Divine Glory has shone from East to West, and through the power of the same Spirit will the divine virtues of humanity become manifest.
(`Abdu’l-Baha: Paris Talks, p. 87)

In the teaching of Baha’u’llah, it is written: `By the Power of the Holy Spirit alone is man able to progress, for the power of man is limited and the Divine Power is boundless.’ The reading of history brings us to the conclusion that all truly great men, the benefactors of the human race, those who have moved men to love the right and hate the wrong and who have caused real progress, all these have been inspired by the force of the Holy Spirit.
The Prophets of God have not all graduated in the schools of learned philosophy; indeed they were often men of humble birth, to all appearance ignorant, unknown men of no importance in the eyes of the world; sometimes even lacking the knowledge of reading and writing.
That which raised these great ones above men, and by which they were able to become Teachers of the truth, was the power of the Holy Spirit. Their influence on humanity, by virtue of this mighty inspiration, was great and penetrating.
The influence of the wisest philosophers, without this Spirit Divine, has been comparatively unimportant, however extensive their learning and deep their scholarship.
The unusual intellects, for instance, of Plato, Aristotle, Pliny and Socrates, have not influenced men so greatly that they have been anxious to sacrifice their lives for their teachings; whilst some of those simple men so moved humanity that thousands of men have become willing martyrs to uphold their words; for these words were inspired by the Divine Spirit of God! The prophets of Judah and Israel, Elijah, Jeremiah, Isaiah and Ezekiel, were humble men, as were also the apostles of Jesus Christ.
Peter, the chief of the apostles, used to divide the proceeds of his fishing into seven parts, and when, having taken one part for each day’s use, he arrived at the seventh portion, he knew it was the Sabbath day. Consider this! and then think of his future position; to what glory he attained because the Holy Spirit wrought great works through him.
We understand that the Holy Spirit is the energizing factor in the life of man. Whosoever receives this power is able to influence all with whom he comes into contact.
The greatest philosophers without this Spirit are powerless, their souls lifeless, their hearts dead! Unless the Holy Spirit breathes into their souls, they can do no good work. No system of philosophy has ever been able to change the manners and customs of a people for the better. Learned philosophers, unenlightened by the Divine Spirit, have often been men of inferior morality; they have not proclaimed in their actions the reality of their beautiful phrases.
The difference between spiritual philosophers and others is shown by their lives. The Spiritual Teacher shows His belief in His own teaching, by Himself being what He recommends to others.
An humble man without learning, but filled with the Holy Spirit, is more powerful than the most nobly-born profound scholar without that inspiration. He who is educated by the Divine Spirit can, in his time, lead others to receive the same Spirit.
I pray for you that you may be informed by the life of the Divine Spirit, so that you may be the means of educating others. The life and morals of a spiritual man are, in themselves, an education to those who know him.
Think not of your own limitations, dwell only on the welfare of the Kingdom of Glory. Consider the influence of Jesus Christ on His apostles, then think of their effect upon the world. These simple men were enabled by the power of the Holy Spirit to spread the glad tidings!
So may you all receive Divine assistance! No capacity is limited when led by the Spirit of God!
The earth of itself has no properties of life, it is barren and dry, until fertilized by the sun and the rain; still the earth need not bewail its own limitations.
May you be given life! May the rain of the Divine Mercy and the warmth of the Sun of Truth make your gardens fruitful, so that many beautiful flowers of exquisite fragrance and love may blossom in abundance. Turn your faces away from the contemplation of your own finite selves and fix your eyes upon the Everlasting Radiance; then will your souls receive in full measure the Divine Power of the Spirit and the Blessings of the Infinite Bounty.
If you thus keep yourselves in readiness, you will become to the world of humanity a burning flame, a star of guidance, and a fruitful tree, changing all its darkness and woe into light and joy by the shining of the Sun of Mercy and the infinite blessings of the Glad Tidings.
This is the meaning of the power of the Holy Spirit, which I pray may be bountifully showered upon you.
(`Abdu’l-Baha: Paris Talks, pp. 163-166)

And now you, if you act in accordance with the teachings of Baha’u’llah, may rest assured that you will be aided and confirmed. In all affairs which you undertake, you shall be rendered victorious, and all the inhabitants of the earth cannot withstand you. You are the conquerors, because the power of the Holy Spirit is your assistant. Above and over physical forces, phenomenal forces, the Holy Spirit itself shall aid you.
(`Abdu’l-Baha: Power of Divine Assistance, p. 213)

He will come to your aid with invisible hosts, and support you with armies of inspiration from the Concourse above; He will send unto you sweet perfumes from the highest Paradise, and waft over you the pure breathings that blow from the rose gardens of the Company on high. He will breathe into your hearts the spirit of life, cause you to enter the Ark of salvation, and reveal unto you His clear tokens and signs.
(`Abdu’l-Baha: Selections … `Abdu’l-Baha, pp. 186-187)

Today, any soul who looseneth his tongue in the delivery of Truth and is engaged in the diffusion of the fragrances of God, he shall undoubtedly be assisted and confirmed by the Holy Spirit and can resist the attacks of all the people of the world, [for the] power of the Realm of Might shall prevail.
(`Abdu’l-Baha: Power of Divine Assistance, p. 211)

But the fourth kind of healing is produced through the power of the Holy Spirit. This does not depend on contact, nor on sight, nor upon presence; it is not dependent upon any condition. Whether the disease be light or severe, whether there be a contact of bodies or not, whether a personal connection be established between the sick person and the healer or not, this healing takes place through the power of the Holy Spirit.
(`Abdu’l-Baha: Some Answered Questions, p. 256)

The healing that is by the power of the Holy Spirit needs no special concentration or contact. It is through the wish or desire and the prayer of the holy person. The one who is sick may be in the East and the healer in the West, and they may not have been acquainted with each other, but as soon as that holy person turns his heart to God and begins to pray, the sick one is healed. This is a gift belonging to the Holy Manifestations and those who are in the highest station.
(`Abdu’l-Baha: Health and Healing, p. 475)

The power of the Holy Spirit, enlightening man’s intelligence, has enabled him to discover means of bending many natural laws to his will. He flies through the air, floats on the sea, and even moves under the waters.
All this proves how man’s intelligence has been enabled to free him from the limitations of nature, and to solve many of her mysteries. Man, to a certain extent, has broken the chains of matter.
The Holy Spirit will give to man greater powers than these, if only he will strive after the things of the spirit and endeavour to attune his heart to the Divine infinite love.
(`Abdu’l-Baha: Paris Talks, pp. 37-38)

From Letters Written on Behalf of Shoghi Effendi:
The quickening power of the Holy Spirit, which has come into the world through Baha’u’llah, is the source of immortal life; and those who are quickened by this spirit in this world will find themselves in great honor and glory in the next world.
(Shoghi Effendi: Guidelines for Teaching, p. 321)