Spiritual life means becoming detached from material life and engaging in the loving service of the Lord. Śrī Caitanya Mahāprabhu therefore asked Rāmānanda Rāya to explain something transcendental to devotional service mixed with speculative knowledge. A pure devotee is completely surrendered to the lotus feet of the Lord, and only by his love does he conquer Kṛṣṇa, who cannot be conquered by anyone.
(Sri Caitanya Caritamrta----2:8:66---purport).
Spiritual life means ānandamaya. There is no displeasure. Always ānanda. That is spiritual life. Spiritual life means ānanda, and material life means anxiety. You can distinguish. If you are always put into anxiety, that is material life. And if you are always jolly, that is spiritual life. Ānandamayo 'bhyāsāt.
(Lecture on Srimad Bhagavatam---1:2:20---31th. October, 1972).
If we understand nicely, na vyathayanti, and you are not disturbed by these changes, then saḥ amṛtatvāya kalpate, then he's making progress, spiritual progress. That means, spiritual progress means, he's making progress towards eternal life. Spiritual life means eternal, blissful life of knowledge. That is spiritual life.
(Lecture on Bhagavad-Gita---2:15---21th. November, 1972).
Material happiness means sense gratification. This is material happiness. Everyone is materialistic, or materially advanced, means he has got better facility for satisfying the senses. That is material life. And spiritual life means he does not satisfy his own senses, but he satisfies the senses of God. That is spiritual life.
(Lecture on Bhagavad-Gita---13:2---4th. April, 1972).
Being pure means spiritual. Spiritual life means pure life, and material life means contaminated life. Just like we have got this body, material body. This is impure body. Therefore we suffer from disease, we suffer from old age, we suffer from birth, we suffer from death. And in our actual, pure form, spiritual form, there is no such suffering.
(Lecture on Bhagavad-Gita---7:2---11th. September, 1968).
Spiritual life means service to Kṛṣṇa. So if you don't serve Kṛṣṇa, where is spiritual life? That is the difference between material life and spiritual life.
(Lecture on Bhagavad-Gita--9:5---24th. April, 1976).
Spiritual life means to become pure, and chanting Hare Krishna will automatically make you pure.
(Letter to Elaine---1st. February, 1976).