The duty of the Guru is to find the means, according to the time, the circumstances and the candidate, by which one can be induced to render devotional service, which Krsna accepts from a candidate who wants to be successful in going back home, back to Godhead.
This makes it easier for the candidate to reach the ultimate destination.
The acaryas´s duty, therefore, is to find the means by which devotees may render service according to references from sastra.
The acarya give the suitable method for crossing the ocean of nescience by accepting the boat of the Lord´s lotus feet, and if this method is strictly followed, the followers will ultimately reach the destination, by the grace of the Lord.
This method is called acarya sampradaya.
We are very satisfied that this path is being opened by this Kṛṣṇa consciousness movement because so many European and American boys and girls are taking this philosophy seriously and gradually attaining perfection.
Śrīmad-Bhāgavatam Canto 5, CHAPTER 14, Purport