SB 4.24.76

ya imaṁ śraddhayā yukto
 mad-gītaṁ bhagavat-stavam
adhīyāno durārādhyaṁ
 harim ārādhayaty asau
Word for word: 
yaḥ — anyone; imam — this; śraddhayā — with great faith; yuktaḥ — devoutly attached; mat-gītam — the song composed by me or sung by me; bhagavat-stavam — a prayer offered to the Supreme Personality of Godhead; adhīyānaḥ — by regular study; durārādhyam — very difficult to worship; harim — the Supreme Personality of Godhead; ārādhayati — he can, however, worship Him; asau — such a person.
Translation: 
Although rendering devotional service to the Supreme Personality of Godhead and worshiping Him are very difficult, if one vibrates or simply reads this stotra [prayer] composed and sung by me, he will very easily be able to invoke the mercy of the Supreme Personality of Godhead.
Purport: 

It is especially significant that Lord Śiva is a pure devotee of Lord Vāsudeva. Vaiṣṇavānāṁ yathā śambhuḥ: “Amongst all Vaiṣṇavas, Lord Śiva is the topmost.” Consequently Lord Śiva has a sampradāya, a Vaiṣṇava disciplic succession, called the Rudra sampradāya. At the present moment those who belong to the Viṣṇusvāmi sampradāya of Vaiṣṇavas come from Rudra, Lord Śiva. To become a devotee of Lord Kṛṣṇa, Vāsudeva, is very, very difficult. The word especially used in this connection is durārādhyam. The worship of the demigods is not very difficult, but becoming a devotee of Lord Vāsudeva, Kṛṣṇa, is not so easy. However, if one adheres to the principles and follows in the footsteps of the higher authorities, as advised by Lord Śiva, one can easily become a devotee of Lord Vāsudeva. This is also confirmed by Prahlāda Mahārāja. Devotional service cannot be practiced by a mental speculator. Devotional service is a special attainment which can be acquired only by a person who has surrendered unto a pure devotee. As confirmed by Prahlāda Mahārāja, mahīyasāṁ pāda-rajo-’bhiṣekaṁ niṣkiñcanānāṁ na vṛṇīta yāvat: “Unless one accepts the dust of the lotus feet of a pure devotee, who is free from all material contamination, one cannot enter into the devotional service of the Lord.” (Bhāg. 7.5.32)