The Covenant and our spiritual development

What is the spiritual foundation upon which the whole Cause of God rests? “...knowledge and appreciation of the Covenants of both Bahá'u'lláh and `Abdu'l-Bahá ... the spiritual foundation upon which the whole Cause of God rests. . . " (Extract from a letter on behalf of Shoghi Effendi to an individual believer dated April 15, 1949, Lights of Guidance, 2nd edition, p. 182, Paragraph 597) What is the relationship between firmness in the Covenant and spiritual development? “The thing the [...] believers must do is to deepen themselves in the Covenants of Bahá'u'lláh and `Abdu'l-Bahá. Mere intellectual understanding of the teachings is not enough. Deep spirituality is essential, and the foundation of true spirituality is steadfastness in the Covenant.” (Shoghi Effendi's secretary on his behalf, letter dated 31 March 1949, "The Light of Divine Guidance," Volume 2, p. 83) What qualities characterize the believers who are firm in the Covenant? " . . . inwardly rejoicing . . . is the characteristic of every soul who is firm in the Covenant." (`Abdu'l-Bahá, Memorials of the Faithful, p. 51) What power animates those who stand identified with, and loyally carry out the provisions and injunctions of, the Will and Testament of `Abdu'l-Bahá? "... the unconquerable power and invincible spirit which animate those who stand identified with, and loyally carry out the provisions and injunctions of, the Will and Testament of `Abdu'l-Bahá." (Shoghi Effendi, This Decisive Hour, Messages to the North American Baha'is 1932-1946, paragraph 85.3, p. 65; Messages to America 1932-1946, p. 49) In order for the teaching work to progress, what must the believers have more than anything else? “The Guardian ...feels that what the [...] Bahá'ís need--and must have--more than anything else in the world is a far deeper understanding of the Covenants of both Bahá'u'lláh and the Master. This is the rock foundation without which no sound super-structure can be built. Neither the administration, nor the general teaching work of the Cause in [...], will progress, or be able to accomplish anything, unless the believers are truly firm, deep, spiritually convinced Bahá'ís.” (From a letter on behalf of Shoghi Effendi, The Light of Divine Guidance, Volume 2, p. 83) What is the greatest of all protections for the Cause of God from its enemies? "The Cause of God must be protected from the enemies of the Faith, and from those who sow seeds of doubt in the hearts of the believers, and the greatest of all protections is knowledge..." (From a letter on behalf of Shoghi Effendi, "The Light of Divine Guidance," Volume I, pp. 134-136) "The greatest protection to the Faith will … be through … ongoing deepening of the Bahá'í community in the Covenant and the history and Teachings of the Faith." (International Teaching Center, letter to the Continental Counsellors dated February 24, 1995)