Many aspiring practitioners of and newcomers to the Vajrayāna come from backgrounds where religion can be studied, practiced, and lived solely through books. This has led to the idea that the same is true for the Vajrayāna, but this is not the case. The Vajrayāna is a tantric vehicle, meaning it follows the tantric framework in study, practice, and progress. Central to the tantras, whether that be Buddhist, Hindu, or Jain, is the master or ācārya. The ācārya is a qualified teacher trained in tantric practices and has the capacity to guide students along the path. For this, we look to the Susiddhikara Sūtra, which states:
Since all mantras are obtained through him, know that an [ā]cārya is the very basis [of all practice].
Mantras, which are central to the Vajrayāna, are not obtained from just anyone or anywhere. They are obtained from an ācārya, and the ācārya is the "very basis" (i.e., foundation, source) of all practice. The same text warns:
[But] if you recite mantras on your own authority without [their having been conferred] by an upādhyāya (preceptor) or ācārya, you will be exerting yourself in vain and will never obtain any result.
A person cannot pick and choose mantras from books or tantras without it being conferred by a qualified master. We have no authority to do so nor do our masters. Our masters received their mantras from their own masters and so on and so forth. Thus, it is on our masters' authority and the authority of their own masters that we recite mantras, not ourselves nor anyone else.
The Hevajra Tantra, talking about the 'innate' or natural state of mind, states:
No one else can explain the innate,
Nor can it be found anywhere.
It is only known for oneself, based on merit,
By attending to the master’s transmission.
With that said, what are the characteristics of a master? According to Longchen Rabjam's Ocean of Siddhis:
Learned here means being sufficiently learned in the philosophical tenets in general to distinguish one’s own and others’ systems... Freedom from activity means remaining alone in solitude, free from the distractions of the eight mundane concerns or ordinary ideas... Being fully acquainted with the meaning of reality (dharmatā) means being well-versed in the meaning of the direct realization of reality and applying it in practice... Not confused as to the points of inquiry means being capable of answering any questions that students might ask.
The Gurupañcāśikā written by Aśvaghoṣa, mentions:
A disciple with sense should not accept as a guru someone who lacks compassion or is prone to anger, vicious, arrogant, possessive, undisciplined or boastful of his knowledge.
Going back to the Susiddhikara Sūtra, it states:
... he always abides by the Dharma and does not do anything that is contrary to the Dharma; he is endowed with great compassion and takes pity on sentient beings... he always wishes for the Great Vehicle and understands the subtle meanings [of its teachings]; he has deep faith in the gateway to the secrets [of the esoteric teachings]... because he seeks the grand [objective of enlightenment], he does not wish for any minor objectives (i.e., mundane siddhis)... his virtue has been extolled by his teacher [with these words]: 'You may go now, having proven yourself fit to be conferred initiation and become an ācārya' (i.e., his own master has authorized him to teach)...
By relying on such a master, the same text states:
... if you rely on such a person, the mantras received [from him] will quickly bring success: of this you should not have any doubts.
These are the important points regarding having a master in order to practice the Vajrayāna. Anyone who claims otherwise in the face of both tantras and commentaries by reputable masters from time immemorial is deluded and is misleading those who are sincerely seeking liberation. Practice and enter the Vajrayāna properly and one will see results in one's practice.