Out of many millions of wandering living entities, one who is very fortunate gets an opportunity to associate with a bonafide spiritual master by the grace of Krishna. By the mercy of both Krishna and the spiritual masters, such a person receives the seed of the creeper of devotional service.
When a pure devotee or spiritual master speak, what he says should be accepted as having been directly spoken by the Supreme Personality of Godhead in the Parampara System. — Sri Caitanya Caritamrita, Antya Lila, 05.71.
The spiritual master is the mercy representative of the Lord.
Srimad Bhagavatam 1.7.22,
The spiritual master and Krishna are two parallel lines. The train, on two tracks, moves forward. The spiritual master and Krishna are like these two tracks, they must be served simultaneously. Krishna helps one to find bona fide spiritual master and bona fide spiritula Master helps one to understand Krishna. If one does not get bona fide spiritual master, then how he can ever understand Krishna? You cannot serve Krishna without spiritual master, or serve just spiritual master without serving Krishna. They must be served simultaneously. — Letter to Mahapurusha, 12th Feb, 1968.
The genuine Guru is God's representative and he speaks about God and nothing else.