Initiation is mentioned at several places in the hymns of the Gurus. To obtain initiation or to be an initiate is the same thing. Muslim holy men use the words 'Beyet Hona' (getting initiated). The Master gives to the disciple His spiritual light. He makes him fit to lead a spiritual life. 

The Gurus describe it as the 'Gift of Life'.

He gives the Gift of Life; He teaches devotion to the Lord. He unites His disciple with the Lord.

Suhi M5, 749-2

Initiation can be had from a Living Master only, and not from the so-called preceptors who merely whisper a mantra in the ears of the disciples. This Gift of Life or Ray of lLfe can be obtained from a living Guru. The life-giving impulse can be obtained only from a living being; it is impossible to get it from books or scriptures.

One meets a Master, and gets initiation. He surrenders all, and enjoys the Inner Life.

 Gauri M1, 227-16

At the time of initiation, the Master imparts the secret knowledge of the heart. It then becomes possible to make spiritual progress by following the Master's directions. After explaining everything, directions are given which help in repetition, contemplation and manifestation of the Inner Shabd or Divine Sound. They help the disciple in his inner ascent. At the time of initiation the Master gives the Ray of Life and connects him with the melody of the Shabd. He then establishes a subtle link with the disciple, guides him and takes him to the original Home. 

However learned, religious-minded, self-disciplined or respected a man may be, he remains deluded by the mind and Maya. 

In these circumstances only a Master can connect him with the inner Shabd. However virtuous a man may be, he cannot, by his own efforts alone, contact the Sound. Unless the Master gives him the gift of the Ray of Life and initiates him, he cannot contact the Divine Sound.

At the time of initiation by the Master the soul becomes fit for being connected with the Shabd. This moment is considered as the moment of birth in the Master's family. After a disciple is connected with the Name by the Master, he begins to progress on the spiritual path and to get control over his weaknesses. When the seeker progresses on the spiritual path by following the directions of the Master, he does not remain subservient to the body. 

On the other hand, his soul tends to soar to the spiritual regions, to break off all the ties with the earth and to dwell more and more in the higher regions within.

This gift of the Name is given to a conscious being, and is meant to awaken him to a new life by its animating impulse. How soon one obtains the benefits of initiation depends on the person initiated. It depends on his inner spiritual condition. The inner condition is different in different persons. Certain persons are fully ready, while others are less so. The Master 'injects', as it were, his consciousness and Light into the soul of the disciple at the time of initiation. This injection of His own Consciousness and Light permeates the disciple like leaven, and produces a new spiritual consciousness and light as the practice of Naam is continued. The spiritual light of the Master gives a new life to the soul of the disciple and begins to free it. Consequently, the disciple begins to have a feeling of fulfilment. This gift of the Master cannot be taken away. Time and Maya (illusion) cannot destroy this seed, this ray of life. The disciple, once initiated, will certainly progress, sooner or later. He will one day certainly reach his true Home.

Unless one gets initiation, one does not get knowledge and therefore cannot discern the truth. 

The Guru says:

There is no knowledge without initiation by a Master.

 Bhairon M5, 1140-15

What is this initiation? It is a Ray of the Name or power of God which can be obtained only from a perfect Master.

Those only  meet a Master, who are so destined. The Name of the Lord is the Nectar; It is given at initiation.

 Suhi M1, 726-15

This initiation is obtained only by great good fortune. When one gets it, the mind becomes happy and still. We know ourselves and are released from birth and death. Initiation from those who have never gone inward, serves no purpose whatever.

This is the Age when men pretend to be pure but are engaged in sins; spiritual pretenders go about offering initiation; the woman loves her husband only for what he earns and cares not where he goes or when he returns; no one accepts the authority of scriptures or Vedas; everyone worships his own self; the judge who occupies the seat of justice, counts the beads and utters the name of God, accepts bribes and deals injustice, and is ready to quote chapter and verse when his conduct is questioned.

 Ramkali Var M1, 51-14

This world of attachment and Maya is like a dangerous ocean in which everyone is floundering. Only a true devotee can cross it. A disciple who gets initiation but practises other rites and austerities and does not give up attachment to the world, is not released from birth and death, and he takes a long time to reach his original Home.

Everyone is floundering in attachment; only a true devotee gets free; while others take birth again because of attachment, and visit the abode of death because of attachment. One may get initiation, but if one recites scriptures and practises austerities, and cannot give up attachment he is not accepted. Attachment is removed through Grace alone; oh Nanak! then alone one merges with the Lord.

 Asa M1, 356-8