The Upanishads are quite clear and emphatic regarding the truth of spiritual life. The spiritual life has only one ideal, the realization of the Supreme Spirit, Brahman, God. The theoretical and intellectual knowledge of God is not enough; in fact it is of secondary importance in our spiritual endeavor. What is of supreme importance in our spiritual aspiration is that each of us must attain the realization of God within his own effulgent cosmic pure consciousness, for thus alone we reach Kaivalyam, we become the absolute One.
Foreword to Kaivalya Upanishad by Swami Premananda
Then the true devotee of self-realization, in whom all personal and objective desires have ceased to exist, approached the great Master with veneration and said: Venerable Sir, teach me the knowledge of Brahman, the one and the absolute God and also the way to attain to the realization of that transcendental Spirit whom the wise constantly seek after, who is hidden in all and by the light of whom one, in due time, gains complete freedom from all subjective finiteness and limitations and becomes one with the supreme Self.
The venerable Master said to him: Seek to realize Brahman by unconditional and abiding faith, supreme devotion, profound meditation and by the practice of the spiritual yoga, the subjective communion of mind with self and self with Self-Consciousness.
The self-disciplined and enlightened devotees who have rightly recognized that scriptural learning is not sufficient to receive spiritual illumination and who have purified their minds and hearts, thoughts and emotions by the practice of subjective renunciation of sensory desires and ego, they, without being distracted by the limitations of the concept of time, meditating on the attributes of God and thereby becoming one with the eternal Brahman, attain to self-liberation.
By perceiving the self in all beings and all beings in the self the devotee attains to the realization of Brahman.
Kaivalya Upanishad (translation by Swami Premananda)
Included in Eight Upanishads by Swami Premananda