The Supreme Importance of the Guru in Sankaradeva's System

The human body, as we have indicated in a previous blog post, is an instrument for realizing pure devotion (bhakti) and, as a by-product of it, salvation (mukti). It is an instrument for educating oneself regarding the true nature of reality--about the external (material) world as well as our own selves--and for feeling the grace of the Lord. For the embodied pure-personality (jiva) to finally take sole-refuge (eka-sarana) in Him and lead a life of pure devotion to God. From an evolutionary perspective, the human body is the most appropriate accompaniment to the transcendental person, now fallen into the material world, for the development of his highest consciousness.
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):

yavata puruse tattva nakare bicara

ajnana timire pari dekhe andhakara

ehimate karmmara adhina huya nara

nanana yonita bhrami phure nirantara

mai basudevata nakare yave mati

yavata sarana laiya nakare bhakati

tavadeke deha dhari bhrame samsarata

janiya bhajiyo mora carana padmata

As long as the embodied pure-personality (purusa) does not contemplate the entities (tattva),

so long does he grope in the darkness of ignorance (ajnana).

In this manner, subservient to karma's will,

man roams endlessly from womb to womb.

As long as he does not direct his mind to me, Vasudeva [the immanent supreme soul of all creatures],

as long as he does not do devotion (bhakti) to me, taking sole-refuge (sarana),

so long will he be traveling in samsara inside a body.

Knowing this truth, you engage exclusively in pure devotion at my lotus-feet.

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):

hari yena ati krpamaya bhakta gurujano sehinaya

duyojana eka sarirata matra bhinna

krparase yukta huya citta lokara hitaka cinti nita

nija gune tusta duyo ahamkare hina.

Like Hari who is extremely full of grace, the devotee-guru is also of the same kind. 

Both are the same, being different only in body. 

Hearts satisfied by the juice of compassion, thinking at all times, the good of the people,

they are by nature satisfied; both are bereft of ego.

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:

vedara rahasya tattva sakale jananta

krsnaka bhakati bole saksata karanta

krsnaka bhakati hove samsaraka tare

nahi nahi samsararta guru ata pare

ehente uttama guru kahilo samprati

iharase upadese suddha haiba mati

amata adhika guru jana sito jana

sisya huya seva karo tahara carana

[Krsna says to Arjuna:] He knows all the mysterious truth (rahasya tattva) of the Vedas

and also, he meets Krsna in person by virtue of his devotional prowess.

[One] accomplishes (pure) devotion to Krsna and crosses the cycle of births and re-births (samsara).

[For this purpose] there is no other, no other Guru apart from him in this world.

He is the best (uttama) guru, this I tell you presently.

And it is only through his instruction (upadesa) that one's intellect will be purified.

Know that one to be a guru greater than me.

Even I become the disciple of such a one and do service to his feet.