Some quotes from HPB & Purucker: 

Self-doomed to live through future Kalpas, unthanked and unperceived by man; wedged as a stone with countless other stones which form the "Guardian Wall", such is thy future if the seventh gate thou passest. Built by the hands of many Masters of Compassion, raised by their tortures, by their blood cemented, it shields mankind, since man is man, protecting it from further and far greater misery and sorrow.

The "Guardian Wall" or the "Wall of Protection." It is taught that the accumulated efforts of long generations of Yogis, Saints and Adepts, especially of the Nirmanakayas -- have created, so to say, a wall of protection around mankind, which wall shields mankind invisibly from still worse evils. [Voice of the Silence]

...Brother of the Light or a Brother of the Shadow. The former is prevented from doing the good he might, by racial, national, and individual Karma; the second is impeded in his fiendish work by the joint efforts of the human “Stones” of the “Guardian Wall.” [CW 12, 603]

July 14, 1936

The Cross of Initiation

G. de P. -- The matter of the trained human ego's having to go through initiation unaided and unprotected by its higher parts is one of great importance, and in the atmosphere of the thought that has been aroused by the various observations made, I want to add something of my own. It is perfectly true that we can receive help from others, for love is always permissible, and it is the sustaining power because it is like health and like fresh air. But when the test comes we have to take it alone. The initiate is in the universe, a part of it, the pulses of the universe are pulsing through him. He has the help of the universe precisely because it is a part of him. But no one helps him by holding him up, or wiping the sweat or dew from his forehead, or giving him injections, etc. The soul must stand naked before the test, and conquer by its own inner powers. If it succeeds, it succeeds, and we have an adept. If it fails, there are other chances, but he gets no help in a direct way whatsoever. If he did, it would be no test. He must learn to fit himself to be one of the stones in the Guardian Wall. The position is one of such heavy responsibility, one involving so much, that there can be no weak links. There is not only no favoritism, there is absolutely no pity shown for the naked soul. If pity were shown, and the soul were helped over and allowed to succeed by supporting hands, the result would be a weak vessel, incapable of holding its own against the terrific impact of cosmic forces outside. The Guardian Wall is composed of human and spiritual stones which are strong in every fiber and have proved themselves to be such before they can be builded into the wall.

You have also heard the statement that the Masters have made in the same connection, that the Guardian Wall guards humanity against evils of which it has no cognizance. What are these evils? They are simply entities and powers and influences and forces which, were they to enter our human sphere, would wreak evil upon us because they are not coordinate with us. They are not in sympathy with us, though in themselves just as good as we are. In their own spheres, in their own lands, they are no more evil than we are. If a human being, for instance, could go into some other planet he might be a worker of evil there, because he would disturb the harmonious relationship prevailing on that other planet. He might be considered a demon, and in a sense he would be there because he would be out of harmonious relationship, out of tune, with the new environment. Consequently the Guardian Wall on such hypothetical planet has to protect that planet against the ingress of impacting monads. Then we should be evil there, not because we are essentially bad, but because we should bring disharmony into that new environment. We should be out of tune there. [Dialogues]

Then, third, comes the event of the Summer Solstice, at which time the neophyte or aspirant must undergo, and successfully prevail over, the greatest temptation known to man just referred to; and if he so prevail, which means the renouncing of all chance of individual progress for the sake of becoming one of the Saviors of the world, he then takes his position as one of the stones in the Guardian Wall. Thereafter he dedicates his life to the service of the world, without thought of guerdon or of individual progress -- it may be for aeons -- sacrificing himself spiritually in the service of all that lives. For this reason the initiation at this season of the year has been called the Great Renunciation. [Four Sacred Seasons]

Should the Mahatma abandon his physical body and live in his other principles, he becomes de facto a Nirmanakaya, living in the auric atmosphere of the earth and working for mankind invisibly, thus becoming one of the living stones in the Guardian Wall.

The Nirmanakaya is a complete man possessing all the principles of his constitution, except the linga-sarira, and its accompanying physical body. He lives on the plane of being next superior to the physical plane, and his purpose in so doing is to save men from themselves by being with them, and by continuously instilling thoughts of self-sacrifice, of self-forgetfulness, of spiritual and moral beauty, of mutual help, of compassion, of pity. Thus it is that he forms one of the stones in the Guardian Wall invisibly surrounding mankind.

Most Mahatmas prepare to become Buddhas of Compassion, and therefore to renounce a nirvanic state.

The real Buddha of Compassion renounces nirvana for himself in order to help the world, for he is compassion incarnate. He lives through aeons, working for all that is, advancing steadily by self-devised efforts, by self-directed evolution, towards divinity, towards godhood; and it is this utter self-sacrifice of the human being, of the most sublime and lofty type conceivable to men, which makes of a Buddha so holy and exalted a being.

The Buddha stands higher even than an Avatara, for the Buddha is a self-chosen incarnation of wisdom and compassion, pity, love, self-forgetfulness. Sons of the Sun, the Buddhas enlighten wherever they go. They abide through the ages and form a Guardian Wall around mankind, protecting it against cosmic perils, of which perils none but high initiates know. The Lords Buddha are the holiest ones. [Golden Precepts]

I might add that the members of the Brotherhood are continuously acting as a Guardian Wall shielding, protecting, mankind from dangers of cosmic and terrestrial character. The method used by these Great Ones is to 'sing' these dangers away from us, through the akasa -- by sound. Now please do not misinterpret these words and picture a row of the Great Teachers standing there, clad in white garments, and opening their mouths and yelling and bellowing and squawking and squealing, as we human beings sometimes do and speak of it as singing! The singing may be utterly soundless to our ears, but it is a song, a song of which the mystics spoke, charming the evil away -- charming means here to sing, to chant, to 'sound' it away.

Now what are these dangers? Let us not think for a moment that they are only material. No; they are of many kinds: spiritual, intellectual, psychical, astral, physical. They are cosmic dangers reaching our earth from outside, from other planets of the solar system, and from dead planets, our moon in particular, and the so-called Eighth Sphere or Planet of Death. Indeed, there are in constant movement along the circulations of the solar system, rivers of lives having as much right to be as we have; but inimical to us in our present evolutionary state, or dangerous at least, and which, if they could enter our earth's atmosphere and impinge upon us, would wipe out the human race overnight. Not a single human being would be left alive upon the earth when morning came.

We humans are pilgrims; we are not forever and unto eternity chained to our globe, nor linked to our planetary chain. We are transients here, although our sojourn is of an exceedingly long time period when judged by human standards. Consequently, there are dangers arising even in this planetary chain, and therefore in globe D of it, which could be highly inimical to man's well-being were they allowed to impinge upon us without a shield or barrier of some sort. These are the terrestrial hazards, and they are of many kinds and on all planes.

Incidentally, one of the greatest of these that we human beings are facing in our own era is the psychical crazes which are sweeping over the world, distracting our minds and diverting us from the thoughts of spirit, of spirituality. This is a psychical danger, which is fraught with tremendous perils because it can mislead human souls.

But let us remember always that we are protected by song, by sound, albeit the most powerful sounds are those which are inaudible to us. The sounds which the human ear can take in, on account of its present imperfections, are but a small part of the octaves of vibration. The greater by far is the inaudible sound. The Music of the Spheres, for instance, is so immense that our ears cannot take it in. Every smallest atom sings its keynote of life as it lives. It is thus by sound, inaudible to us, that it grows; it is thus by sound, inaudible to us because of its magnitude, that the planets and the suns follow their pathways of destiny, and weave their web of being and so grow to grander things.

Sound is a form of radiation. Radiation is but a form of sound. It was no vain way of speaking, when some of the ancient European peoples told us that their ancient magicians sang the storms away, sang diseases away, sang men into health and sanity, sang men into goodness and wisdom. [ET 5, 72-4]