When a living entity desires to serve the Lord in transcendental loving service, the Lord helps the devotee in so many ways as the caitya-guru, or the spiritual master within, and thus the devotee can perform many wonderful activities beyond material estimation.
(Srimad Bhagavatam---3:9:38---purport).

The Lord is the Supersoul seated in everyone's heart, and He acts as the caitya-guru, the spiritual master within. However, He gives direct instructions only to the advanced, pure devotees. In the beginning, when a devotee is serious and sincere, the Lord gives him directions from within to approach a bona fide spiritual master.
(Srimad Bhagavatam--4:28:41---purport).

One who is sincere and pure gets an opportunity to consult with the Supreme Personality of Godhead in His Paramātmā feature sitting within everyone's heart. The Paramātmā is always the caitya-guru, the spiritual master within, and He comes before one externally as the instructor and initiator spiritual master. The Lord can reside within the heart, and He can also come out before a person and give him instructions. Thus the spiritual master is not different from the Supersoul sitting within the heart.
(Srimad Bhagavatam---4:28:52---purport).

It is not possible for a conditioned soul to directly meet Kṛṣṇa, the Supreme Personality of Godhead, but if one becomes a sincere devotee and seriously engages in devotional service, Lord Kṛṣṇa sends an instructing spiritual master to show him favor and invoke his dormant propensity for serving the Supreme. The preceptor appears before the external senses of the fortunate conditioned soul, and at the same time the devotee is guided from within by the caittya-guru, Kṛṣṇa, who is seated as the spiritual master within the heart of the living entity.
(Sri Caitanya Caritamrta---1;1:58---purport).

One should not take any responsibility on his own but should be a soul surrendered to the Supreme Personality of Godhead, who will then give him dictation as thecaittya-guru, or the spiritual master within. The Supreme Personality of Godhead is pleased to guide a devotee from within and without. From within He guides him as the Supersoul, and from without He guides him as the spiritual master.
(Sri Caitanya Caritamrta---1:8:79).