I've heard that the Ishta-Deva is pre-decided for each person.
How do you know who your Ishta-Deva is?
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The key role in revealing one's Ishta Devata plays the Atma Karaka. Atma Karaka is the planet that has the highest longitude. To find one's Ishta Devata we have to look at the Atma Karaka in Navamsa, since the Navamsa shows blessings of God onto the native and the way in which the native communicates with God.
Atma Karaka in Navamsa is called the "Karakamsa". The 12th from the Karakamsa determines the "Istha Devata". The 12th house from the karakamsa is called the "Jivanmuktamsa". This is the house signifying the emancipation of the soul and we have to examine this house in order to find one's Ishta Devata. If there is a graha in the Jivanmuktamsa, it signifies the Devata. If there are more grahas than one, we choose the strongest. If there is no graha in the Jivanmuktamsa, you can see whether any Grahas are aspecting the 12th. If no Grahas aspect it, you can take the 12th Lord as Ishta Devata.
The deities are worshipped based on the planets in the 12th from kaarakaamsha. Here are >the presiding deities for each planet who will be your Ishta Devata as per your horoscope.
- Sun~ Vishnu/ Rama/ Shiva
- Moon ~ Krishna/Parvati
- Mars ~ Hanuman/ Subramanya/ Sri Narasimha
- Mercury ~ Vishnu
- Jupiter ~ Vishnu/ Sri Vamana/ Dattatreya/ Sai Baba
- Venus ~ Mahalakshmi/ Parashurama
- Saturn ~ ShaniDev/ Hanuman/ Kurma
- Rahu ~ Durga/ Varaha
- Ketu ~ Ganesh/ Matsya
- Ascendent ~ Kalki
For example: Sun in 12th from Atma Karaka shows a worshipper of Shiva and Moon in 12th from Atma Karaka shows a worshipper of Gouri.
Source : Ishta Deva
I am answering based on Jaimini Sutras. Here, we found what particular celestial configurations result into devotion to a particular diety. From Ch1, Pada 2,
There are other predictions as well when one worships evil spirits. One should note that if one has more messy configuration then other aspects need to be taken into consideration.
There are three possible ways, in my opinion, that Ishta Devata is chosen by a spiritual aspirant. These three ways are:
(1) The family deity ultimately becomes Ishta Devata;
(2) A spiritual aspirant becomes attracted to a particular Deity after reading through different spiritual literature and accepts that Deity his Ishta Devata; and
(3) The Guru of the spiritual aspirant meditates and finds out about the person's Ishta Devata.
Ishta comes from the word Ish - to desire. The Ishta (Chosen Ideal) is the form of God that the aspirant desires.
Ramakrishna Paramahamsa says (Gospel of Sri Ramakrishna, Chapter 8, also here: http://www.belurmath.org/gospel/chapter08.htm)
MR. CHOUDHURY: "Sir, is it not possible to have the vision of God without the help of a guru?"
MASTER: "Satchidananda Himself is the Guru. At the end of the Shavasadhana, just when the vision of the Ishta is about to take place, the guru appears before the aspirant and says to him, 'Behold! There is your Ishta.' Saying this, the guru merges in the Ishta. He who is the guru is also the Ishta. The guru is the thread that leads to God. Women perform a ritualistic worship known as the 'Ananta-vrata', the object of worship being the Infinite. But actually the Deity worshipped is Vishnu. In Him are the 'infinite' forms of God.
(To Ram and the other devotees) "If you asked me which form of God you should meditate upon, I should say: Fix your attention on that form which appeals to you most; but know for certain that all forms are the forms of one God alone.
Your Ishta is the form that appeals to you most; if you are not sure your guru can confirm who your ideal is.
Swami Vivekananda says (Complete Works, V4, Addresses on Bhakti Yoga, The Ishta, also here: http://cwsv.belurmath.org/volume_4/vol_4_frame.htm):
This theory of Ishta, therefore, means allowing a man to choose his own religion. One man should not force another to worship what he worships...
So in religion — more so — I must teach myself religion. What right has my father to put all sorts of nonsense into my head? What right has my master or society to put things into my head? Perhaps they are good, but they may not be my way. Think of the appalling evil that is in the world today, of the millions and millions of innocent children perverted by wrong ways of teaching. How many beautiful things which would have become wonderful spiritual truths have been nipped in the bud by this horrible idea of a family religion, a social religion, a national religion, and so forth. Think of what a mass of superstition is in your head just now about your childhood's religion, or your country's religion, and what an amount of evil it does, or can do. Man does not know what a potent power lies behind each thought and action. The old saying is true that, "Fools rush in where angels fear to tread." This should be kept in view from the very first. How? By this belief in Ishta. There are so many ideals; I have no right to say what shall be your ideal, to force any ideal on you. My duty should be to lay before you all the ideals I know of and enable you to see by your own constitution what you like best, and which is most fitted to you. Take up that one which suits you best and persevere in it. This is your Ishta, your special ideal.
Ishta Deva can be found using 12th house of Swamsa chart or Karakamsha chart as explained in Brihat Prashara Hora Shastra as follows
If the 12th from Karakamsa has a benefic, the expenses will be on good account while a malefic will cause bad expenses. If the said 12th be vacant then also good effects (in respect of expenses) will follow. If there be a benefic planet in exaltation or in own house in the 12th from Karakamsa or if Ketu is so placed and aspected by or conjunct a benefic one will attain heavens after death. One will attain final emancipation if Ketu is in the 12th (from Karakamsa) identical with Aries/Sagittarius and is aspected by a benefic. If Ketu is in the 12th from Karakamsa and be with aspect/conjunction of a malefic one will not attain final emancipation.
If the Sun and Ketu are in the said 12th the native will worship Lord Shiva. The Moon and Ketu in the said 12th denotes a worshipper of Gauri (a consort of Lord Siva). Venus and Ketu in the said 12th denotes a worshipper of Lakshmi (a consort of Lord Vishnu) and a wealthy person. Mars and Ketu in the said 12th denotes a worshipper of Lord Subramanya (an offspring of Lord Siva). Rahu in the 12th from Karakamsa will make one worship Durga or some mean diety. Ketu alone in the said 12th house denotes Subramanya's or Ganesa's worshipper. If Saturn is in the said 12th in a malefic's sign one will worship mean deities. Venus and Saturn in the said 12th in a malefic's sign will also make one worship mean deities. Similar inferences can be drawn from the 6th Navamsa counted from Amatya Karaka's Navamsa.
Ketu in/with Benefics(Moon, Jupiter, Venus and Mercury, rest are natural malefics) in 12th house of Svamsa Karakamsa can be seen in the horoscopes of sages Ramana Maharshi, Paramhansa Yogananda signifying they were in liberated state and ascended to higher realms eventually.