When transcendental devotional service, by which love for Krishna is attained is executed by the senses, it is called Saadhana Bhakti, or the regulative discharge of devotional service. Such devotion eternally exists within the heart of every living entity. The awakening of this eternal devotion is the potentiality of devotional service in practice. — Sri Caitanya Caritamrta, Madhya Lila, 22.105.
Devotional service to Krishna is the chief function of the living entity. There are different methods for the liberation of the conditioned soul Karma, jnana, yoga and Bhakti but all are dependent on bhakti. — Sri Caitanya Caritamrita, Madhya lila, 22.17.
The manifestation of unadulterated devotional service is exhibited when one’s mind is at once attracted to hearing the transcendental name and qualities of the Supreme Personality of Godhead, who is residing in everyone’s heart.
Srimad Bhagavatam 3.29.11-12,
One who acts to serve Krishna with his body, mind, intelligence and words is a liberated soul, even within this material world.