Three Ways to Enter Grace
One can enter grace in three ways: 1) by oneself, without intermediary, when it is awakened spontaneously. This process is denoted by the term pratibhijnanin, [a] the gnostic, master by illumination; 2) through the intermediary of scriptures, wherein a man takes from the scriptures a formula which guides him to liberation; 3) with the help of a teacher.
[a. Pratyabhijna is translated as direct knowledge of one's self; recognition. en.wikipedia.org/wiki/
If Reality is always present, and if grace resides in all beings and can manifest spontaneously, why does Siva reveal himself through teachers and scriptures? Abhinavagupta responds quite simply that this is his wish. He reveals himself both as guru and disciple, both as the facilitator of the awakening and the awakening itself. The teacher does not manifest Reality, he simply frees the student from his ignorance and his misconceptions of duality. "With the sword of initiation, sever the bonds which were once attached to me; immediately Reality shone forth, just as a smoldering fire bursts into flame when the ashes are poked." (XIII, 175).
Among the masters themselves, we can distinguish the initiate – trained by one or many masters – from the pratibho guru who has received illumination without intermediary. But insofar as the disciple is concerned, each may be called sadguru, an authentic master.
Pratibho Guru, The Master Through Illumination
Such a master has experienced an intense grace which, pouring over him, has immediately dissolved his ignorance. He absorbs himself into the divine energy, and of his own accord understands the nature of bondage and liberation. He then resolutely attaches himself to the supreme essence, losing himself in Siva and identifying with Him. Liberated while alive, his only task 'here below' is to deliver others.
This illuminating intuition, which acts as an inner teacher, springs from the depths of the Self and depends neither on scriptures nor an 'outer' guru. From the start, it must be held onto. One must disregard all other knowledge, just as one extinguishes a lamp when the sun rises (XIII, 179). In effect, it alone permits the absolute certainty which ends the doubts which are inseparable from duality. Of what importance is it if this jnanin has not been initiated in the regulations? He possesses true mastery, having received the initiation from his own inner energies when they all converged effortlessly at their center and plunged into undifferentiated Consciousness (1-1).
It behooves this eminent master to accept the help of another master or to re-examine the scriptures so that he might become still more perfect and more certain of his own convictions. He is thereafter called truly accomplished because of the fullness of his triple illumination – the spontaneous illumination joined with the illumination born of his scriptural study together with that of his teacher, with which he finally identifies. In the course of deep samadhi, he has first of all diligently exerted himself, joining his guru's wisdom with his own. Following his teacher through a complete range of mystical experiences, he inherits the vast knowledge accumulated over the centuries by a lineage of qualified masters (IV, 76-77).
But if illumination is instantaneous, how is it that it can also be developed? In fact, illumination's depth and breadth vary indeed, and it does not always occur in precisely the same manner. Sometimes it strikes like a brilliant flash and never returns. Or, initially vacillating, it may become stronger by degrees, with or without the help of a teacher. Sometimes it appears definitively right from the start, making its "possessor" a pratibho guru.
Thus all masters do not exercise the same sphere of influence: illumination surges forth in some only enough for their own liberation; it permits others to liberate a small number of disciples; it renders a few rare individuals capable of liberating a multitude of disciples. A universal master liberates all of humanity. One can compare them, respectively, to a glow worm which shines only inwardly for himself, to a jewel, to a star, to the moon, and to the sun (XIII, 159 and IV, 139).
One can recognize this pratibho guru by specific signs of which the most important is an unshakeable devotion to Siva. Master of the mantras and the powers which they contain, he also dominates the elements, finds success in all his enterprises, possesses a poetic gift, and understands all the scriptures.
These various powers surge forth when intuitive discrimination reveals itself. His organs and his thoughts purified (and thus perfectly conscious), he can hear and see at a distance. But he ought not to use these powers except to encourage faith and certitude in his disciples. In this way he will inspire in them the confidence which will aid in their liberation. (XIII, 183).
The Authentic Teacher (Sadguru)
The teacher deserving of this name should have perfect Self-knowledge (atman) and should be identified with Siva. Abhinavagupta defines such a teacher beautifully when he describes him as having "ardent vigilance."
Indifferent to the opinion of others, he shuns all ostentatiousness, unlike the hypocrite who presents himself as an illuminated master all the while lacking knowledge of the Self. And the deluding energy of which the hypocrite is victim, is produced, like grace, independent of merit or fault. In this, Siva, through his freedom, totally disimilates his own nature and ridicules his proper role; he could not go further in this play! An awakened being, because of his free consciousness, may comport himself publicly like an ignoramus since he scoffs at social convention. Likewise, an ignoramus under the influence of divine energy may act like a guru in a most serious manner, even if he feels only scorn for his comportment. For neither does he confuse himself with his role. His lack of sympathy for his own behavior condemns he who has not received grace (XIV, 6-8).
Further, it is necessary to distinguish carefully a true guru from jnanins and yogins who have not overcome tendencies towards duality and for this reason, do not constitute true masters. There exist gnostics filled with knowledge but lacking in mystical experience, and there are yogi technicians skilled in experience but without knowledge.
According to the Kashmir Shaivites, yogins and jnanins are roughly divided into four groups: to the first group belongs the theologian who is concerned only with theory and can teach the revealed texts which he has studied and understood. Similarly, the yogin initiated in yoga devotes himself to various practices. From the second group arises the gnostic who has a living experience of the treatises, having understood them in their deepest sense through intuitive discrimination (cinta). Since his well-exercised intuition permits him to know the Self, he becomes a zealous yogi who pursues the yogic path with ardor and dedicates his life to it.
But only the gnostic of the third group is a true master,  for he possesses the complete mystical experience which at its summit rejoins unwaivering illumination, the essential knowledge of the divine. Even if he has not read the treatises, he knows them instinctually. He is, furthermore, a master in samadhi, from which derives his name, siddhayogin, "the accomplished yogin." Though he has rejected all duality, he retains enough awareness of the distinction between guru and sisya [disciple]  to accomplish his work of liberation. From this order arises a potent master capable of giving numerous disciples knowledge of the Self as well as supernatural yogic powers. Abhinavagupta is perhaps the finest example of such a master.
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There exists still another group of yogins and jnanins, by nature very perfect (susiddha). Since they live in undifferentiated samadhi without interruption, for them everything is absolute fullness. They perceive neither bondage nor liberation and cannot, consequently, assume responsibility for disciples (XIII, 328).
A scripture defines the guru's fundamental task: "'0 Beloved,' declares Siva to the Goddess, 'he who, from the scriptures or from the master's words discovers what water and ice are, has no further tasks to accomplish; this birth will be his last.'" The guru, in melting the heart of the disciple, returns to their fluid state the parcelled and hard ice blocks of his thoughts. The disciple in confidence allows himself to be carried by the waters of undifferentiated life, following the guru's and Siva's subtle instigations. Both confidence and surrender are on his part indispensible. He cannot participate in mystical knowledge and in the guru's power if he does not absorb himself totally in the guru. This surrender leads first to identification with the master, then with Siva. By the grace of the Self, one's artificial personality is utterly destroyed. Nevertheless, the disciple does not become a slave: he obeys only that which is his essential nature and discovers freedom in the very heart of his most complete surrender.