In the Bhagavad-gita Lord Krishna unequivocally declares that He is the Supreme Absolute Truth and that it is the duty of everyone to render Him loving devotional service. The Bhagavad-gita was revealed for the sole purpose of explaining these two principal points. One who understands them is eligible to begin spiritual life as a neophyte devotee.
His Divine Grace A. C. Bhaktivedanta Swami Prabhupada
Renunciation Through Wisdom 1.9

Only that person whom Lord Krishna sends us as spiritual master will manifest before us as our guru. By the Lord’s mercy we attain a spiritual master, and by the spiritual master’s mercy we attain Krishna. We are given a spiritual master according to our fortune.
Srila Bhaktisiddhanta Sarasvati Thakura
Amrita Vani, Section: The Spiritual Master

Devotional service is so strong that when one engages in it, he gradually gives up all material desires and becomes fully attracted to the lotus feet of Krishna. All this is brought about by attraction for the transcendental qualities of the Lord.
Sri Chaitanya Mahaprabhu
Sri Chaitanya-charitamrita, Madhya-lila 24.198

Let the wise worship and serve Lord Govinda, the infallible Supreme Personality of Godhead, who lifts His devotees from the mud of material sufferings, takes them to His eternal and blissful spiritual abode, and out of deep love for them will not leave them for even half a moment.
Sri Baladeva Vidyabhushana
Govinda Bhashya on Vedanta-sutra, Epilogue

The Supreme Personality of Godhead is so kind to the conditioned souls that if they call upon Him by speaking His holy name, even unintentionally or unwillingly, the Lord is inclined to destroy innumerable sinful reactions in their hearts. Therefore, when a devotee who has taken shelter of the Lord’s lotus feet chants the holy name of Krishna with genuine love, the Lord can never give up the heart of such a devotee. One who has thus bound the Supreme Lord’s lotus feet within his heart with the ropes of love is to be known as bhagavata-pradhana, the most exalted devotee of the Lord.
Sri Havis Rishi
Srimad-Bhagavatam 11.2.55

By the Supreme Lord’s mercy the soul’s ancient knowledge is revived.
Shvetashvatara Upanishad 4.18

The Supreme Personality of Godhead is the life of all. He is the Supersoul splendidly manifest in all living beings. One who knows Him becomes wise. That person turns from the logicians’ debates. He meditates on the Lord’s pastimes. He loves the Lord. He serves the Lord. He is the best of transcendentalists.
Mundaka Upanishad 3.1.4