Atrinetrah sarvasaksi
Acaturbahur acyutah,
Acaturvadano brahma
Sriguruh kathitah priye
O dear one, it is true that Shri Guru is (Shiva), the witness of all, but without three eyes; he is Vishnu without four arms; he is Brahma without four faces.
In continuation with the previous sloka, Lord Shiva elaborates on the truth that the in word Guru Gu symbolises guna and Ru denotes rupa. Yet the Guru is nirakara. Even though he does not seem to have 3 eyes physically, the Guru is Shiva with three eyes. Even though the Guru does not seem to have four arms like Lord Vishnu is depicted with four arms, the Guru is Lord Vishnu, and although he does not have four heads just as Lord Brahma is illustrated, the Guru is Lord Brahma.
In this verse, Lord Shiva is clearly giving us the message that we should dissolve the external appearance of the Guru or the form of the Guru within us. Instead of giving prominence only to external worship, we must internalise the Guru. We must see the shakti or power of the Guru, not the rupa or the external form. The Guru reflects his Knowledge or Jnana as Brahma, his Chaitanya or consciousness as Vishnu and Sadhana or Tapas as Shiva. Thus, in this sloka, Lord Shiva says, Guru is the form of the trinity. To this Supreme Guru, we must bow down. Thus, when we see the Guru as knowledge, he manifests the creative energy of Brahma, When we feel him as Chaitanya or consciousness he reveals himself as the sustaining force of Vishnu and when we invoke the Guru in our spiritual practices or tapas he becomes Lord Shiva Himself.
No matter which form we implore, we must always remember that the Guru is Jagadoddharaka. The world is still existing today only because the Guru’s presence. He is the witness of all our thoughts, speech and actions. Therefore Lord Shiva says he is sarva saakshi. For instance, if we take the case of an acting competition, the judge or the jury of judges, only award points to the participants based on the performance of the participants. They are not biased about any participant based on any pre-conceived grounds.
Likewise, in the exam of our life, the Guru is the sole witness and valuator of our actions. He awards us marks or points based on our performance. If we perform dharma, we earn dharmic points and therefore we will be blessed with good karma if not vice versa.
Excerpt from The Force of Nirvana by Shri Shri Nimishananda