SB 4.24.55
tam duraradhyam aradhya
satam api durapaya
ekanta-bhaktya ko vañchet
pada-mulam vina bahih
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
My dear Lord, pure devotional service is even difficult for liberated persons to discharge, but devotional service alone can satisfy You. Who will take to other processes of self-realization if he is actually serious about the perfection of life?
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The word satam refers to transcendentalists. There are three kinds of transcendentalists: the jñani, yogi and bhakta. Out of these three, the bhakta is selected as the most suitable candidate to approach the Supreme Personality of Godhead. It is emphasized herein that only one who is outside devotional service would not engage in searching for the lotus feet of the Lord. Foolish people sometimes maintain that God may be attained in any way — either by karma-yoga, jñana-yoga, dhyana-yoga, etc. — but here it is clearly stated that it is impossible to obtain the mercy of the Lord by any means but bhakti-yoga. The word duraradhya is especially significant. It is very difficult to attain the lotus feet of the Lord by any method other than bhakti-yoga.