"When Sanatana Gosvami asked Sri Caitanya Mahaprabhu about the svarupa of every living being, the Lord replied that the svarupa or constitutional position of the living being is the rendering of service to the Supreme Personality of Godhead. If we analyze this statement of Lord Caitanya's, we can easily see that every living being is constantly engaged in rendering service to another living being. A living being serves other living beings in two capacities. By doing so, the living entity enjoys life. The lower animals serve human beings as servants serve their master. A serves B master, B serves C master and C serves D master and so on. Under these circumstances, we can see that one friend serves another friend, the mother serves the son, the wife serves the husband, the husband serves the wife and so on. If we go on searching in this spirit, it will be seen that there is no exception in the society of living beings to the activity of service. The politician presents his manifesto for the public to convince them of his capacity for service. The voters therefore give the politician their valuable votes, thinking that he will render valuable service to society. The shopkeeper serves the customer, and the artisan serves the capitalist. The capitalist serves the family, and the family serves the state in the terms of the eternal capacity of the eternal living being. In this way we can see that no living being is exempt from rendering service to other living beings, and therefore we can safely conclude that service is the constant companion of the living being and that the rendering of service is the eternal religion of the living being.
Yet man professes to belong to a particular type of faith with reference to particular time and circumstance and thus claims to be a Hindu, Muslim, Christian, Buddhist or any other sect. Such designations are non-sanatana-dharma. A Hindu may change his faith to become a Muslim, or a Muslim may change his faith to become a Hindu, or a Christian may change his faith and so on. But in all circumstances the change of religious faith does not affect the eternal occupation of rendering service to others. The Hindu, Muslim or Christian in all circumstances is servant of someone. Thus, to profess a particular type of sect is not to profess one's sanatana-dharma. The rendering of service is sanatana-dharma.
"Factually we are related to the Supreme Lord in service. The Supreme Lord is the supreme enjoyer, and we living entities are His servitors. We are created for His enjoyment, and if we participate in that eternal enjoyment with the Supreme Personality of Godhead, we become happy. We cannot become happy otherwise. It is not possible to be happy independently, just as no one part of the body can be happy without cooperating with the stomach. It is not possible for the living entity to be happy without rendering transcendental loving service unto the Supreme Lord."
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.