The tenets of the Zoroastrian religion are basically contained in the 'Gathas'. Gathas are sacred songs divided into stanzas and strophes. The five Gathas are :
Gatha Ahunavaiti, on freedom of choice.
Gatha Ushtavaiti, on supreme bliss (ushta)
Gatha Spenta Mainya, on the holy spirit
Gatha Vohu Kshathra, the gatha of the Good Kingdom
Gatha Vahishtoishti, the gatha of sovereign desire or the gatha of fulfillment.
The word 'Ahu' means life, energy, the moving force which operates in Nature, so the 'Ahunavaiti Gatha' deals with the laws which operate in nature with freedom of choice, the creation of the world and the origin of evil. The second Gatha, the 'Ushtavait' leads the reader to religious ecstasy showing the way to deep joy and happiness. The 'Spenta Mainya Gatha' deals with the essence of the absolute, the spirit which permeates all existence. It unfolds the perfect wisdom which activates the Divine Plan. Man cannot comprehend this perfection and wholeness until he develops his own mental faculties and acquires moral strength. This is the gist of the fourth Gatha, the 'Vohu Kshathra'. The fifth Gatha, the 'Vahishtoishti', is the gatha of fulfillment where in a series of admonitions are laid down for men to follow.
With Ahura Mazda, the supreme Creator and ruler with the Universe, six other great beings are associated in the Gathas. These Beings were later named the Amesha Spenta, Holy immortals or Immortal Shinning Ones. These six Divine Powers or Energies are:
Asha, the Divine Law, truth, Righteousness and Justice.
Vohu mana, the Good Mind, representing divine Love that transcends all bounds of time and space.
Kshathara, the power and Majesty of Ahura Mazda, his choice Kingdom.
Kshthra-Vairya, the creative activity in the service of mankind.
Aramati, Divine Devotion, stability, loyality, the love of God in our hearts.
With Ahura mazda at their head ,the seven Amesha Spenta are collectively known as the Hafta Amahashpands.
The language in which the Gathas are written is termed as Gathie Avestan. It bears a strong affinity with Rig-Vedic Sanskrit. The Avesta language is believe to have came into existence some three hundred years after Zarathushtra.
The Gathas were written when Zarathushtra was meditating in a mountain probably Demavand whereas other Avesta were written by his disciples in Eastern Iran .Yashts which is long poems of praise and Nyaeshes which is shorter poems of praise were composed during this period.
Zarathushtra preached his religion in the Kyanian period of Iranian history. After the overthrow of that dynasty, there was a sudden revival of the Zorashtrian religion. The omnibus book called Yasna or Yezashne is the collection of Gathas and Avesta writings. The Visperad and Vendidad are writings of this period.
The entire library at Persepolis, containing a mammoth collection of books, manuscripts and other writings of learnings along with the entire Avesta writings were burnt to ashes by Alexander the Great. The Zoroastrian scriptures were once again neglected and disjoined until another great Zoroastrian, Ardeshir Papakan founded the last Zoroastrian empire in the Iran called Sassanian empire. It was during this period that a concise prayer book was composed called the 'Khorda Avesta', or 'Khordeh Avesta'. It contains Avestan prayers and passages in Pazand with commentaries on astronomy, medicine and cosmogony.
Zarathushtra insisted upon active good towards one's fellow man, the motto of the religion became "Good thoughts, good words, good deeds".