He whose heart is purified can see that the whole cosmic manifestation is but the Supreme Personality of Godhead, but he whose heart is contaminated sees things differently. Therefore by sat-saṅga, or association with devotees, one becomes perfectly pure in heart.
(Srimad Bhagavatam---4:24:59---purport).

One who is pure in heart is never attracted by the external energy, which urges the individual soul to try to dominate material nature.
(Srimad Bhagavatam---4:24:59---purport).

The pure heart of a devotee is never disturbed when he executes devotional service in the form of hearing, chanting, remembering, etc. In all, there are nine processes one can follow in the execution of devotional service. In any case, a pure-hearted devotee is never disturbed.
(Srimad Bhagavatam---4:24:59---purport).

Persons who have been thoroughly cleansed by devotional service and are always joyful, being situated in elevated consciousness, who are very much attached to the studies of Śrīmad-Bhāgavatam, who are always cheerful in the association of devotees, who have accepted the lotus feet of Kṛṣṇa as the ultimate shelter of their lives, and who are pleased to perform all details of devotional service, have in their pure hearts the transcendental ecstasy of attachment.
(Teachings of Lord Caitanya).

Devotees have no need to compete with materialists because they are on the path back to Godhead, back home where everything is eternal, full and blissful. Such transcendentalists are a hundred percent nonenvious and are therefore pure in heart.
(Teachings of Lord Caitanya).

The sun is far above the planetary systems, and there is no possibility of its being covered by any kind of cloud; similarly, when a devotee is purified like the sun, from his pure heart there is a diffusion of ecstatic love which is more glorious than the sunshine.
(Nectar of Devotion).