Preaching is the best service to the Lord.

“The Lord says in Bhagavad-gita (18.55), bhaktya mam abhijanati yavan yas casmi tattvatah: “”One can understand the Supreme Personality as He is only by devotional service.”” Prahlada Maharaja ultimately instructed his class friends, the sons of the demons, to accept the process of devotional service by preaching the science of Krishna consciousness to everyone. Preaching is the best service to the Lord. The Lord will immediately be extremely satisfied with one who engages in this service of preaching Krishna consciousness. This is confirmed by the Lord Himself in Bhagavad-gita (18.69). Na ca tasman manusyesu kascin me priya-krttamah: “”There is no servant in this world more dear to Me than he, nor will there ever be one more dear.”” If one sincerely tries his best to spread Krishna consciousness by preaching the glories of the Lord and His supremacy, even if he is imperfectly educated, he becomes the dearmost servant of the Supreme Personality of Godhead. This is bhakti. As one performs this service for humanity, without discrimination between friends and enemies, the Lord becomes satisfied, and the mission of one’s life is fulfilled. Sri Caitanya Mahaprabhu therefore advised everyone to become a guru-devotee and preach Krishna consciousness (yare dekha, tare kaha ‘Krishna’-upadesa). That is the easiest way to realize the Supreme Personality of Godhead. By such preaching, the preacher becomes satisfied, and those to whom he preaches are also satisfied. This is the process of bringing peace and tranquillity to the entire world.

bhoktaram yajna-tapasam sarva-loka-mahesvaram
suhrdam sarva-bhutanam jnatva mam santim rcchati

One is expected to understand these three formulas of knowledge concerning the Supreme Lord—that He is the supreme enjoyer, that He is the proprietor of everything, and that He is the best well-wisher and friend of everyone. A preacher should personally understand these truths and preach them to everyone. Then there will be peace and tranquillity all over the world. The word sauhrdam (“”friendliness””) is very significant in this verse. People are generally ignorant of Krishna consciousness, and therefore to become their best well-wisher one should teach them about Krishna consciousness without discrimination. Since the Supreme Lord, Vishnu, is situated in the core of everyone’s heart, every body is a temple of Vishnu. One should not misuse this understanding as an excuse for such words as daridra-narayana. If Narayana lives in the house of a daridra, a poor man, this does not mean that Narayana becomes poor. He lives everywhere–in the houses of the poor and those of the rich–but in all circumstances He remains Narayana; to think that He becomes either poor or rich is a material calculation. He is always sad-aisvarya-purna, full in six opulences, in all circumstances.”

Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Seventh Canto, Chapter 06 – Text 24