Perfection means to regain one’s original spiritual form and engage in the loving service of the Lord. The living entity is part and parcel of the Supreme Lord, and if he performs the duties of the part and parcel, without proudly thinking he is one in all respects with the Supreme Lord, he attains real perfection and becomes peaceful.
His Divine Grace A.C. Bhaktivedanta Swami Prabhupada
Narada-bhakti-sutra 4, Purport

When an intelligent person engages constantly in worshiping Me through loving devotional service as described by Me, his heart becomes firmly situated in Me. Thus all material desires within the heart are destroyed.
Lord Sri Krishna
Srimad-Bhagavatam 11.20.29

Because a devotee of Lord Krishna is desireless, he is peaceful. Fruitive workers desire material enjoyment, jnanis desire liberation, and yogis desire material opulence; therefore they are all lusty and cannot be peaceful.
Sri Chaitanya Mahaprabhu
Sri Chaitanya-charitamrita, Madhya-lila 19.149

Do not stay in illusion; go to the eternal reality. Do not stay in darkness; go to the light. Do not keep taking material bodies; become immortal!
Brihad-aranyaka Upanishad 1.3.28

One who engages in the transcendental service of the Lord in body, mind, and word is to be considered liberated in all conditions of material existence.
Srila Rupa Goswami
Bhakti-rasamrita-sindhu 1.2.187

For a person devoid of devotional service, birth in a great family or nation, knowledge of revealed scripture, performance of austerities and penance, and chanting of Vedic mantras are all like ornaments on a dead body. Such ornaments simply serve the concocted pleasures of the general populace.
Hari-bhakti-sudhodaya 3.12

When a living entity is conditioned by material nature, he is exactly like a person haunted by a ghost.
Jagadananda Pandita
Prema Vivarta 1

Without the unconditional mercy of the Lord’s most beloved friends, a devotee’s loving service can never succeed or bear fruit.
Sri Hanuman
Brihad-Bhagavatamrita 1.4.86