CC Ādi 1.58

jīve sākṣāt nāhi tāte guru caittya-rūpe
śikṣā-guru haya kṛṣṇa-mahānta-svarūpe
Synonyms: 
jīve — by the living entity; sākṣāt — direct experience; nāhi — there is not; tāte — therefore; guru — the spiritual master; caittya-rūpe — in the form of the Supersoul; śikṣā-guru — the spiritual master who instructs; haya — appears; kṛṣṇa — Kṛṣṇa, the Supreme Personality of Godhead; mahānta — the topmost devotee; sva-rūpe — in the form of.
Translation: 
Since one cannot visually experience the presence of the Supersoul, He appears before us as a liberated devotee. Such a spiritual master is none other than Kṛṣṇa Himself.
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.