Therefore the Personality of Godhead and His different incarnations and forms played the parts of devotees to instruct the conditioned souls how to approach the transcendental stage of devotional service. Advaita Acarya especially intended to teach the conditioned souls about devotional service. The word acarya means "teacher." The special function of such a teacher is to make people Krsna conscious. A bona fide teacher following in the footsteps of Advaita Acarya has no other business than to spread the principles of Krsna consciousness all over the world. The real qualification of an acarya is that he presents himself as a servant of the Supreme. Such a bona fide acarya can never support the demoniac activities of atheistic men who present themselves as God. It is the main business of an acarya to defy such imposters posing as God before the innocent public.
(Sri Caitanya Caritamrta----1:6:28----purport).

But who is acarya? Acarya means one who has received the Vedic knowledge through the parampara system. Evam parampara-praptam imam rajarsayo viduh (B.g.4:2). He is acarya. Acarya cannot be manufactured, self-made. No. He must come down from the disciplic succession. He is acarya. You have to approach such acarya who is coming in disciplic succession. acaryavan puruso veda. One who is under the shelter of acarya, he knows things. So Krsna recommends also here that if you actually want to become in knowledge, if you want to have real knowledge, then you must worship acarya. Acaryopasanam. This is Vedic system.
(Lecture on Bhagavad-Gita----13:8-12).

The bona fide spiritual master is called ācārya, or the ācārya can become a bona fide spiritual master. Ācārya means who knows the purport of the sāstra, and he behaves himself according to the shastric regulative principle and teaches his disciple in that way. He is called ācārya. Ācārya means whose behavior, whose activities should be followed. That is called ācārya.
(Lecture on Srimad Bhagavatam----1:2:2).