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.
Devotional Service is just like a seed sown in the heart of a living entity and if he goes on hearing & chanting, that seed fructifies, just as the seed of a tree fructifies with regular watering. Thus the spirtitual plant of devotional Service grows gradually.
Pure devotional service is the satisfying activity for the soul. So Lord Caitanya was specifically preaching pure devotional service, this was his unique contribution. Even in the history of whole vedic culture it was very rare to find someone who is preaching pure devotional service, other than Narada-muni and some acharyas. Lord Caitanya was very strict he wanted his followers to maintain that integrity of preaching only pure devotional service. When some of his followers visit other people who are not preaching pure devotional service, he would abandon them.
Although karma, jnana and yoga are discussed in Bhagavad-gita, the only purpose is to inform us of Krishna’s greatness and engage us in His devotional service.
Lecture on "Understanding Krishna through Bhagavad Gita" – Salem – 8th Nov'06
Loving devotional service to the Lord begins with hearing about the Lord.
Srimad Bhagavatam 1.7.7,