Vivekachudamani 15

ato vicarah kartavyo jijñasoratmavastunah ||
samasadya dayasindhum gurum brahmaviduttamam || 
(Sankaracarya's Vivekachudamani 15)

Therefore, a true seeker of the Self should proceed with his inquiry after duly approaching a Master who is established in the experience of the Self and who is an ocean of compassion.

The Supreme Personality of Godhead is already in the core of everyone’s heart. Isvarah sarva-bhutanam hrd-dese ’rjuna tisöhati (Gita 18.61). The causeless mercy of the Supreme Personality of Godhead is first appreciated when one comes in touch with a bonafide spiritual master who can bring the conditioned soul to the highest position of devotional life. Therefore Lord Caitanya said that by the mercy of the spiritual master one can achieve the causeless mercy of the Lord, and by the mercy of the Supreme Personality of Godhead, one can attain the mercy of the bonafide spiritual master. 

Guru is the friend of the fallen, he is the giver of bliss and the only ocean of mercy.