The more one advances in spiritual consciousness, the more he can understand the elements of devotional service. Nothing in devotional service is material; everything is spiritual. Consequently a devotee is awarded so-called material opulence for spiritual advancement. This opulence is an aid to help the devotee advance toward the spiritual kingdom. – His Divine Grace A. C. Bhaktivedanta Swami Prabhupada Srimad-Bhagavatam 6.16.29, Purport

The atma [individual soul] is called all pervading (sarva-gatah) because it accepts all types of bodies, such as that of man, devata, bird, and beast, one after another, according to karma. The atma also possesses a fixed form (sthanuh) and fixed qualities (acalah). – Srila Baladeva Vidyabhusana Bhagavad-gita 2.24, Commentary

Among the followers of Vedic knowledge, most are following the process of fruitive activity and distinguishing between good and bad work. Out of many such sincere fruitive actors, there may be one who is actually wise. Out of many millions of such wise men, one may actually become liberated [mukta], and out of many millions of such liberated persons, a pure devotee of Lord Krishna is very difficult to find. – Sri Caitanya Mahaprabhu Sri Caitanya-caritamrta, Madhya-lila 19.147–148

As long as we hanker after our own pleasures, we try to enjoy the world through the senses and are given to hollow argumentation. But this world is not made for our enjoyment. When spiritual bliss will appear in us like the incessant flow of oil, then shall we be truly tied to the feet of Krishna. – Srila Bhaktisiddhanta Sarasvati Thakura Lecture, Vrindavan, 1928

Since maintaining one’s life is not possible without action, one must work to maintain his life. If such activities are done in the mood of enjoyment, then one’s qualification as a human is lost and he becomes like an animal. Therefore if one can transform all his bodily activities into activities favorable to the devotional service of the Lord, then that is bhakti-yoga. – Srila Bhaktivinoda Thakura Sri Bhaktyaloka, Part 1 (Atyahara)

Scriptural evidences clearly indicate that for the yogis, jnanis, and karmis to become successful in their individual paths, their engagement in devotional service is imperative, whereas to attain perfection on the path of bhakti, which results in spontaneous love of Godhead (prema), there is no need of yoga, jnana, karma, or any other process. – Srila Visvanatha Cakravarti Thakura Madhurya-kadambini, First Shower of Nectar