"Sri Rupa Gosvami affirms herein that there are nine different kinds of devotional service, which are listed as hearing, chanting, remembering, serving, worshiping the Deity in the temple, praying, carrying out orders, serving Krishna as a friend and sacrificing everything for Him. Each and every one of these processes is so powerful that if anyone follows even one single one of them, he can achieve the desired perfection without fail. For example, if one is attached simply to hearing about the Lord and another is attached to chanting the glories of the name, both will achieve their desired goal in devotional service. In the Caitanya-caritamrta this has been explained. One may execute one, two, three or all the different processes of devotional service, and at the ultimate end he will achieve the desired goal of being established in devotional service."
(Nectar of Devotion, Chapter 14)
"All living entities are part and parcel of Krishna, and therefore when they revive their original Krishna consciousness, they possess all the good qualities of Krishna in a small quantity. When one engages himself in the nine processes of devotional service (sravanam kirtanam visnoh smaranam pada-sevanam/ arcanam vandanam dasyam sakhyam atma-nivedanam (SB 7.5.23), one's heart becomes purified, and he immediately understands his relationship with Krishna. He then revives his original quality of Krishna consciousness."
(Srimad-Bhagavatam 5.18.12, Purport)
"There are instances in history of great personalities, including sages and kings, who attained perfection by this process. Some of them attained success even by adhering to one single item of devotional service with faith and perseverance. Some of these personalities are listed below.
1. Emperor Pariksit attained the spiritual platform simply by hearing from such an authority as Sri Sukadeva Gosvami.
2. Sri Sukadeva Gosvami attained the same simply by recitation, verbatim, of the transcendental message which he received from his great father, Sri Vyasadeva.
3. Emperor Prahlada attained spiritual success by remembering the Lord constantly, in pursuance of instructions given by Sri Narada Muni, the great saint and devotee.
4. Laksmiji, the goddess of fortune, attained success simply by sitting and serving the lotus feet of the Lord.
5. King Prthu attained success simply by worshiping the Lord.
6. Akrura, the charioteer, attained success simply by chanting prayers for the Lord.
7. Hanuman (Mahavira), the famous nonhuman devotee of Lord Sri Ramacandra, attained success simply by carrying out the orders of the Lord.
8. Arjuna, the great warrior, attained the same perfection simply by making friends with the Lord, who delivered the message of Bhagavad-gita to enlighten Arjuna and his followers.
9. Emperor Bali attained success by surrendering everything unto the Lord, including his personal body.
These are nine standard modes of devotional service to the Lord, and a candidate can choose to adopt any one, two, three, four or all, however he likes. All the services rendered to the Absolute are in themselves absolute, with none of the quantitative or qualitative differences found on the material platform. On the spiritual platform everything is identical with everything else, although there is transcendental variegatedness."
(Easy Journey to Other Planets, Chapter 1)
"These nine processes of devotional service can grant one Krishna and ecstatic love for Him. In the beginning one has to discharge devotional service according to regulative principles, but gradually, as devotional service becomes one’s life and soul, one achieves the most exalted position of ecstatic love for Krishna. Ultimately, Krishna is the goal of life."
(Caitanya-caritamrta Antya 4.70, Purport)
"One's sincere endeavor to perform these nine processes of devotional service is technically called bhakti. The word addha means 'directly.' One should not be like the karmis, who perform pious activities and then formally offer the results to Krishna. That is karma-kanda. One should not aspire for the results of his pious activities, but should dedicate oneself fully and then act piously. In other words, one should act for the satisfaction of Lord Visnu, not for the satisfaction of his own senses."
(Srimad-Bhagavatam 7.5.23-24, Purport)
"These are nine processes of devotional service, of which sravanam, hearing, is most important. Without hearing, nobody can understand the science of God. Therefore the Vedic mantras are called sruti. Sruti means it is to be heard. It is not to be experimented in the laboratory. It is simply to be heard. Therefore it is called sruti. So sravanam kirtanam visnoh smaranam pada-sevanam, arcanam vandanam dasyam. Vandanam, offering prayer. We also offer prayer. The other religious sect, just like the Christians, they offer prayer; the Muhammadans, they offer prayer. So prayer, offering prayer is also one of the items of bhakti. Chanting, hearing, meditating, offering prayers, arcanam, worshiping the Deity in the temple, all of them are together devotional service. So out of the nine... If you can execute all the nine, it is very good. But it is not possible. So even if you can execute one item, you become perfect. It is so nice."
(Srila Prabhupada Lecture, Montreal, August 16, 1968)
"This tree of devotional service is not of this material world. It grows in the spiritual world, where there is no distinction between one part of the body and another. It is something like a tree of sugar, for whichever part of such a tree one tastes, it is always sweet. The tree of bhakti has varieties of branches, leaves and fruits, but they are all meant for the service of the Supreme Personality of Godhead. There are nine different processes of devotional service (sravanam kirtanam visnoh smaranam pada-sevanam arcanam vandanam dasyam sakhyam atma-nivedanam (SB 7.5.23), but all of them are meant only for the service of the Supreme Lord. Therefore whether one hears, chants, remembers or worships, his activities will yield the same result. Which one of these processes will be the most suitable for a particular devotee depends upon his taste."
(Caitanya-caritamrta Adi 9.25, Purport)
"Our preaching must be based upon the subject matter from books and nothing outside of them. The actual fact is that worshiping the Deity and chanting the glories of the Lord are both within the nine processes of devotional service recommended by Rupa Goswami. Besides chanting and hearing and remembering there are six other recommended activities in devotional service. So if we are able to practice even only one of these activities perfectly, the result will be the same. Or we may execute more than one, we may execute several or even all nine activities in devotional service, but even if we execute one perfectly we shall be completely successful in devotional service. So there is no question that one activity is more important than another or that Deity worship is more important than Sankirtan, but one individual may be able to perform one activity more satisfactorily than another, so to him that activity will be more important."
(Srila Prabhupada Letter, June 16, 1972)