But mere presence of the human body, although it supplies the congenial environment, would not automatically guarantee the obtaining of the saving knowledge of devotion by the jiva. An external teacher is required in order to educate the jiva and to color his mind with devotion and to connect him to Paramatma, the indwelling preceptor. This requirement is fulfilled by the entity known as Guru whose job it is to instruct the lay people of the world in the matter of obtaining their supreme welfare and to exhort them to take sole-refuge in the Lord.
In the Bhagavata, it is said that as long as a person does not engage in understanding the ontological categories or entities (tattvas), he does not come out of the darkness of ignorance (this passage is from the fifth canto):
It is precisely in order to educate the jivas regarding the tattvas (concerning such questions as which entity is inert or unconscious and which is conscious, etc.) and to direct the mind of the jivas towards God that the Guru makes his appearance. Though, literally speaking (i.e. from the literal point of view), Guru and Deva (God) are two distinct entities, yet, on account of the similarity in their essential nature, they are identified (i.e. considered as one and the same). Madhavadeva writes on this striking similarity between God and Guru in his Nama Ghosa (v. 681):
The Lord and the Guru who gives the knowledge of devotion to the Lord are so similar in their nature (as well as their role) that the two entities are identified. Both are of utterly compassionate nature.
Now, regarding the true Guru, Sankaradeva, in his Bhakti Pradipa, has the following lines to say: