Siddhi for many mantras will happen only if the person who is reciting them undergoes initiation of mantra by a guru.
Does the siddhi of a mantra occur if initiation is done by a guru who is not eligible to initiate?
Are there any explicit statements stating about siddhi to a person who is reciting a mantra after initiation from an ineligible guru?
According to a Vaishnava scripture known as Krishna Bhajanamritam, gurus can be of different spiritual potencies. The etiquette after receiving initiation from a guru who may be of lower potency is described.
Verse 46
If one's spiritual master is of small spiritual potency, or in other words, if he or she does not possess a special power to give spiritual instruction on worship for devotional service, then one may listen from the mouth of other great advanced Vaisnavas and understand the special instructions. However, thereafter, the disciple must go the spiritual master for confirmation on the instructions heard.
Verse 48
Just as a faithful son may go out for earning money and subsequently brings to his father the wealth gained, later the son may ask for some allowance from the father and whatever he receives from the father he is entitled to spend of his own enjoyment. Similarly, a disciple may hear instructions from another advanced Vaisnava but after gaining that good instruction he must bring it and present it to his own spiritual master. After presenting them, he should relish the ganges of those teachings from his or her spiritual master with appropriate instructions.
Verse 55
If a father or spiritual master or husband are not possessed of outstanding qualities even then they are always worshipable.
Verse 59-60
If the spiritual master commits a wrongful act of breaking Vaisnava regulative principles then in that case one should in a solitary place, confront him for his rectification using logic and appropriate conclusions from references to shastras and other gurus, but one is not to give him up.
One should not be hesitant or fearful because one is confronting or challenging a spiritual master, for it has been prescribed that one must appropriately discipline even a spiritual master who is bewildered about what he should or shouldn't do; who is inexperienced or ignorant; who has deviated from the Krsna conscious path; or if he is bewildered by false pride.
Verse 64
However, if the spiritual master acts enviously towards that which is connected with the Supreme; is bewildered regarding the Supreme Personality of Godhead; is averse to expanding the fame of Lord Krishna; personally refuses to accept hearing or chanting about the glorious pastimes of Lord Sri Krsna; has become totally bewildered, listening to the false praise of ignorant persons and day by day is more materially contaminated and fallen, then the spiritual master must be renounced.
Verse 65
Under those circumstances one should not doubt, "How can I give up my spiritual master?" With a strong desire for achieving spontaneous devotional service and attaining the lotus feet of Lord Krsna, the Supreme Personality of Godhead, a devotee accepts the shelter of a spiritual master, if that spiritual master takes on "asuric" qualities or a demoniac mentality then it is one's duty to reject such a guru and > in his place accept a Krsna conscious spiritual master and worship him.
Verse 66
By taking shelter of the strength of the Krsna conscious spiritual master's devotional service the ill effects or contamination of the previous spiritual master is counteracted and destroyed. These activities are recommended by all Vaisnava authorities as the authorized conclusions of the sastra.
Verse 67
During the pastimes of Lord Sri Krsna Caitanya Mahaprabhu there have been many practical examples of the above. This concludes the deliberation and final conclusions of sadhu, sastra on guru and related matters.
In short, Mantra received from a guru who may not be fully self-realized does not go in vain but it depends on several factors as described above.
