Kingdom of Godhead
The Kingdom of Godhead is not a myth but it is situated far beyond the boundary of the material cosmos and one can know it by the process of devotional service only.
Sincere Devotional Service
If one renders even a small amount of sincere devotional Service, Krishna becomes obliged to elevate such person and what to speak of one whose entire life and soul has been dedicated to Krishna’s service. — Letter to Harer Nama, 06th Nov, 1969.
Sadhana Bhakti
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.
The Chief Function of The Living Entity
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
As soon as the body is finished, all material achievements are also finished. No one takes with him anything he has achived materially in this world. The only thing he can carry with him is his asset of devotional service; that alone is “never vanquished”.
Manifestation of Unadulterated Devotional Service
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,
Srila Prabhupada