What is the origin of the Great Invocation ?
The Great Invocation was given to humanity by the spiritual Hierarchy, the aggregate of those members of humanity who have triumphed over matter, who have achieved the goal of self-mastery by the same path that individuals tread today. The spiritual Hierarchy is the result of human activity, aspiration and achievement; it has been created by and of humanity. The members of the Hierarchy have achieved a complete control of the personality or lower self. They are no longer centred in the individualised consciousness but have entered into the wider realisation of the planetary group life, which includes all stages, from that tiny sense of social responsibility of the man or woman taking the first steps upon the path of spiritual maturity, to the inclusive comprehension of the Christ Himself.
The Masters are known as the "Custodians of the Plan". The Plan is the blueprint of divine Purpose, and it is for all people everywhere in the world. Only through humanity can the divine Plan work out, but the Hierarchy works under spiritual law and never coerces or pressures humanity. Individual freedom, particularly mental freedom, is never infringed.
The Plan works out under the impelling power of evolution itself. To hasten humanity's evolutionary development, the Great Invocation was given out in three stages over a period of nine years which included the earthly event of the second phase of the World War. As a result of the collaboration of Alice Bailey and a Tibetan Master of the Wisdom, "seven very ancient word-forms" were translated into "understandable and adequate" phrases in English.
These seven word-forms are said to constitute the complete, new utterance of the Christ for the coming 2000-year epoch, and thus the Great Invocation is spoken of as "the Christ's own mantram". 
What is invocation ?
Invocation and its inevitable response, evocation, describe that mysterious something - emanation, voiceless appeal, inherent urge towards the light - which is innate in all forms, and which is the cause of all progress or pushing forward along the path of expanding consciousness and penetration into the light. This is true of a plant pushing its way out of the darkness of earth into the light of the sun, of a child extricating itself under the life impulse from the womb of its mother, of the human being pushing into realms of greater knowledge, of the aspirant and disciple driving forward on the way of liberation, and of still greater beings penetrating into realms of divine life beyond the human being's comprehension. All comes about through invocation and evocation, appeal and response.
This evolutionary pushing forward out of darkness into light, from the unreal to the real, and from death to immortality, is an inherent urge in all forms. It underlies evolution and the interaction of cause and effect, for always we get what we invoke. The knowledge of this scientific fact will be one of the great liberating forces for humanity. 
What is meant by the door where evil dwells ?
The three aspects or divine potencies of Mind, Love, and Will are anchored in humanity itself - "the centre which we call the race of men". Here and here alone lies the promise of the future, for only here can all the divine qualities express themselves and find fulfillment. Only through humanity, alone and unaided (except by the divine Spirit in every human being), can "the door where evil dwells" be sealed. It is not God who seals that door, nor is it Hierarchy which forces evil back into the place from whence it comes, for the Great Invocation here refers to the evil which finds its origins not on cosmic levels, and therefore beyond man's capacity to resolve, but the evil which is allowed to manifest in human selfishness, hatred, separativeness, and materialism. It is struggling, aspiring, suffering humanity to whom the task of sealing the door is committed and humanity, we are assured, is adequate to the task. The responsibility of Those most skilled in using the Invocation is to focus and anchor the invoked energies of Light and Love and Power in the human kingdom. From that point, humanity can take over the task. 
What is the "race" of men ?
A race in the sense that it is used in the Great Invocation means an evolutionary stage. The race of men is one of three planetary centres - Shamballa, Hierarchy, and Humanity - upon our planet. The close relation of these three centres permits an unbroken series of impressions, relating one centre to another and developing the Science of Invocation and Evocation. Shamballa serves as the head centre - the centre of reception and direction - of our planetary Logos; Hierarchy is the heart centre - the centre of inclusive love, cohesiveness and the circulation of life-bearing energy; and Humanity is the throat centre and the prime creative agency upon our planet.
The outstanding characteristic of the human race at its present stage in evolution is its capacity for intelligent sensitivity to impression. The human race includes those who are sensitive to impression from levels both "above" and "below" the human. As humanity becomes adept at invoking the impression of Hierarchy, the civilisations and cultures created by man will adhere increasingly to the Plan. Here again emerges another reason for the importance of the centre which we call the race of men and an indication of humanity's crisis, for man is now at a point where the intellect is so strongly awakened that nothing can arrest his progress into knowledge which could be dangerously used or selfishly applied if nothing were done to safeguard him. 
What is the intended time span for the Great Invocation?
The Great Invocation is said to have a symbol in the archives of the Masters indicating the era or period in human history during which it can and should be used. Because the Invocation is Christ's own mantram and His complete utterance for the Aquarian age, we can assume that the period of its use will span at least the coming 2000 years. It is a group prayer for all humanity in the Aquarian age and therefore it can be used both as an invocation and as a prayer.
Humanity has reached a critical point in evolution. Human intelligence is now so strongly awakened that nothing can arrest man's progress into realms of knowledge which will be dangerously misused and selfishly applied if nothing is done to safeguard him from himself. Man must be taught to respond to higher, more spiritual values or the increasing stage of integration of many millions of human beings will simply be directed, with more effectiveness, to selfish and materialistic purposes. The form side of manifestation - mind, emotion and brain - must be brought into correspondence with wisdom, love and directed purpose.
It is humanity's responsibility to implement the Plan of Love and Light on Earth. In this we see the opportunity of Triangles, a meditation activity in which groups of three individuals join together in consciousness each day to say the Great Invocation and visualise the energies of light and goodwill radiating throughout the planetary network composed of all Triangles. Through this network in consciousness, the Light, Love and Will of God can stream forth into fuller livingness, fostering the transformation of human lives and establishing an era of right human relations.
The Great Invocation provides, as a result of its correct use, a spiritual inflow right to the very heart of humanity and from the highest sources. In receiving the Great Invocation, in its use and distribution, humanity has been participating in a cosmic event of tremendous importance.
Increasingly in the coming era, disciples and people of spiritual aspiration will grow in their understanding of the potent energy embodied in "Words of Power" such as the Great Invocation. Together with an understanding of ritual, and using the potency of sound, the disciple in the future will work and build the new world with its culture and civilisation. From a study of the tonal values, numerological indicators, and inherent potency of Words of Power such as the Great Invocation, humanity will eventually gain in skill at building the forms and producing the group activities which truly express the Plan. Already we understand that scientific formulas have reduced the most intricate and complex truths to a few signs and symbols. A word or words can also provide "the power of embodiment" for spiritual truths. As the Bible says, "God spoke and the worlds were made". Such is the spiritual potential contained within the Great Invocation when used with precision, understanding of its divine origin, and the soul quality of true humility.