Who is a Guru?

The Guru is God Himself manifesting in a personal form to guide the aspirant. Grace of God takes the form of the Guru. To see the Guru is to see God. The Guru is united with God. He inspires devotion in others. His presence purifies all.

The Guru is verily a link between the individual and the immortal. He is a being who has raised himself from this into That, and thus has free and unhampered access into both the realms. He stands, as it were, upon the threshold of immortality; and, bending down he raises the struggling individuals with his one hand, and with the other lifts them up into the empyrean of everlasting joy and infinite Truth-Consciousness.

A Sadguru is endowed with countless Siddhis (psychic powers). He possesses all divine Aisvarya (powers), all the wealth of the Lord.

The Sadguru is Brahman Himself. He is an ocean of bliss, knowledge, and mercy. He is the captain of your soul. He is the fountain of joy. He removes all your troubles, sorrows, and obstacles. He shows you the right divine path.

He tears your veil of ignorance. He makes you immortal and divine. He transmutes your lower, diabolical nature. He gives you the rope of knowledge, and takes you up when you are drowning in this ocean of Samsara (cycle of birth and death). Do not consider him to be only a man. If you take him as a man, you are a beast. Worship your Guru and bow to him with reverence.

Guru is God. A word from him is a word from God. He need not-teach anything. Even his presence or company is elevating, inspiring, and stirring, His very company is self-illumination.

Gururbrahamaa Gururvishnuh
Gururdevo Maheshvarah
Gurureva Parabrahma
Tasmai shreeguruvave namah

Guru Gita v.32
The Guru is Brahma, He is Vishnu, He is Shiva
Guru is indeed Parabraman, the Supreme Absolute
Salutations to Him!
A Guru is absolutely necessary for every aspirant in the spiritual path. It is only the Guru who will find out your defects. The nature of egoism is such that you will not be able to find out your own defects. Just as a man cannot see his back, so also he cannot see his own errors. He must live under a Guru for the eradication of his evil qualities and defects.
The aspirant who is under the guidance of a Master or Guru is safe from being led astray. Satsanga or association with the Guru is an armour and fortress to guard you against all temptations and unfavourable forces of the material world.
Stick to One Guru
Do not dig here and there shallow pits for getting water. The pits will dry up soon. Dig a very deep pit in one place. Centralise all your efforts here. You will get good water that can supply you throughout the year. Even so, try to imbibe thoroughly the spiritual teachings from one preceptor alone. Drink deep from one man. Sit at his feet for some years. There is no use of wandering from one man to another man, out of curiosity, losing faith in a short time. Do not have the ever-changing mind of a prostitute. Follow the spiritual instructions of one man only. If you go to several people and follow the instructions of many persons, you will be bewildered. You will be in a dilemma.
Dhyaan moolam Guror murtih
Pooja moolam Guror padam
Mantra moolam Guror vyaakam
Moksha moolam Guror kripa
Guru Gita v.76
The root of the meditation is the Guru's form
The root of worship is the Guru's feet
The root of the mantra is the Guru's word
The root of liberation is the Guru's gace

Reverence to the Sadguru who washes away the bonds of karma
Necessity of a Guru

There is no religion on earth which does not revere a guru. Every religion and its revelation have descended from the mouth of the guru and have developed according to his instructions. God Himself is the first teacher of mankind or Jagad-Guru. It is the great teachings of the guru that are written in all the great books and religions. Realized as they are, they and their teachings go together, for speech is the essence of man. Who can fathom the glory of such a God-man? Blessed is the land; blessed are the parents who give birth to such a holy personage.

Lord Krishna sat at the feet of His Guru Sandeepani. Lord Rama had Guru Vasishtha who gave Him Upadesha (spiritual advice). Lord Jesus sought John to be baptised by him on the banks of the river Jordan. Even Devas (celestial beings) have Brihaspati as their Guru. Even the greatest among the divine beings sat at the feet of Guru Dakshinamurti.

"Attain this knowledge by all means; if you prostrate yourself at the feet of the wise, render them all forms of service and question them with a guileless heart, again and again, those wise seers of truth will unfold that wisdom to you."

Bhagavat Gita Ch. 4.34