I now assure thee, O servant of God, that, if thy mind become empty and pure from every mention and thought and thy heart attracted wholly to the Kingdom of God, forget all else besides God and come in communion with the Spirit of God, then the Holy Spirit will assist thee with a power which will enable thee to penetrate all things, and a Dazzling Spark which enlightens all sides, a Brilliant Flame in the zenith of the heavens, will teach thee that which thou dost not know of the facts of the universe and of the divine doctrine. Verily, I say unto thee, every soul which ariseth today to guide others to the path of safety and infuse in them the Spirit of Life, the Holy Spirit will inspire that soul with evidences, proofs and facts and the lights will shine upon it from the Kingdom of God. Do not forget what I have conveyed unto thee from the breath of the Spirit. Verily, it is the shining morning and the rosy dawn which will impart unto thee the lights, reveal the mysteries and make thee competent in science, and through it the pictures of the Supreme World will be printed in thy heart and the facts of the secrets of the Kingdom of God will shine before thee.
(‘Abdu’l-Bahá, Bahá’í World Faith, p. 369)

If some people find that prayer and placing all their trust in God releases in them a flood of inspiration, they should be left free to pursue this method if it is productive of results.
(Shoghi Effendi, Principles of Bahá’í Administration, p. 10)

The fourth standard is that of inspiration. In past centuries many philosophers have claimed illumination or revelation, prefacing their statements by the announcement that “this subject has been revealed through me” or “thus do I speak by inspiration.” Of this class were the philosophers of the Illuminati. Inspirations are the promptings or susceptibilities of the human heart. The promptings of the heart are sometimes satanic. How are we to differentiate them? How are we to tell whether a given statement is an inspiration and prompting of the heart through the merciful assistance or through the satanic agency?
(‘Abdu’l-Bahá, The Promulgation of Universal Peace, p. 254)

The inspiration received through meditation is of a nature that one cannot measure or determine. God can inspire into our minds things that we had no previous knowledge of, if He desires to do so.
(Shoghi Effendi, Principles of Bahá’í Administration, p. 10)

The question arises: How shall we know whether we are following inspiration from God or satanic promptings of the human soul? Briefly, the point is that in the human material world of phenomena these four are the only existing criteria or avenues of knowledge, and all of them are faulty and unreliable. What then remains? How shall we attain the reality of knowledge? By the breaths and promptings of the Holy Spirit, which is light and knowledge itself. Through it the human mind is quickened and fortified into true conclusions and perfect knowledge.
(‘Abdu’l-Bahá, The Promulgation of Universal Peace, p. 22)

We cannot clearly distinguish between personal desire and guidance, but if the way opens, when we have sought guidance, then we may presume God is helping us.
(Shoghi Effendi, Principles of Bahá’í Administration, p. 10)

What is inspiration? It is the influx of the human heart. But what are satanic promptings which afflict mankind? They are the influx of the heart also. How shall we differentiate between them?
(‘Abdu’l-Bahá, The Promulgation of Universal Peace, p. 22)

… through innate ability, practice and study … this is the normal way for inspiration to reach us, through the channels of our own abilities, and not through control by forces which the Master warned us against, and which we do understand, and which—as you yourself know—are neither consistent nor reliable.
(Shoghi Effendi, Spiritualism, Psychic Phenomena and Related Subjects, p. 2)