You will find in the Gita: “Learn thou this by discipleship, by enquiry and by service. The wise, the seers of the essence of things, will instruct thee in wisdom.” (Chapter IV – verse 34.)
The guru must not only be a Brahma-srotriya (well-versed), but a Brahma-nista (established in Brahman) as well. Mere study of books cannot make one a guru. One who has studied the vedas and who has direct knowledge of the atman, through anubhava (direct experience), is a guru. If you find peace in the presence of a holy man (mahatma) and if your doubts are removed in his presence, you can take him as your guru. When the guru gives the mantra to his disciples, he gives with it his own power.
Just as water flows in a river, so also jnana (knowledge) and bhakti (devotion) are ever flowing from a sage. Only a thirsty man drinks water. So too, a thirsty aspirant, who has implicit faith in his guru and is eager to imbibe his teachings, can drink the nectar from him. The student can imbibe from his guru only in proportion to the intensity of his faith in him.
The guru tests the students in various ways. Some students misunderstand him and lose their faith in him. Hence they are not benefited. But those that stand the tests, boldly, come out successful in the end.
The periodical examinations in the ‘University of Sages’ are very stiff indeed. Once a great sage (Gorakhnath) asked some students to climb a tall tree and throw themselves head downwards onto a very sharp trident (trisula). Many of the faithless students kept quiet. But one faithful student at once climbed up the tree with lightning speed and hurled himself down. He was protected by the invisible hand of the sage and had immediate self-realisation. This man had no deha-adhyasa (feeling, “I am body”), but the others had strong attachment for their bodies.
Once Guru Govind Singh tested his students. He said: “My dear disciples, if you have real devotion towards me, let six of you come forward and give me your heads.” Two faithful disciples offered their heads.
Many people debate over the necessity for having a guru. Some assert vehemently that it is not necessary to have a guru for spiritual advancement and that one can attain self-realisation through one’s own efforts, only. No spiritual progress is possible unless a man gets the benign grace and direct guidance of a spiritual preceptor.