SB 11.29.6

naivopayanty apacitiṁ kavayas taveśa
 brahmāyuṣāpi kṛtam ṛddha-mudaḥ smarantaḥ
yo ’ntar bahis tanu-bhṛtām aśubhaṁ vidhunvann
 ācārya-caittya-vapuṣā sva-gatiṁ vyanakti
na eva — not at all; upayanti — are able to express; apacitim — their gratitude; kavayaḥ — learned devotees; tava — Your; īśa — O Lord; brahma-āyuṣā — with a lifetime equal to Lord Brahmā’s; api — in spite of; kṛtam — magnanimous work; ṛddha — increased; mudaḥ — joy; smarantaḥ — remembering; yaḥ — who; antaḥ — within; bahiḥ — outside; tanu-bhṛtām — of those who are embodied; aśubham — misfortune; vidhunvan — dissipating; ācārya — of the spiritual master; caittya — of the Supersoul; vapuṣā — by the forms; sva — own; gatim — path; vyanakti — shows.
O my Lord! Transcendental poets and experts in spiritual science could not fully express their indebtedness to You, even if they were endowed with the prolonged lifetime of Brahmā, for You appear in two features — externally as the ācārya and internally as the Supersoul — to deliver the embodied living being by directing him how to come to You.

According to Śrīla Jīva Gosvāmī, Lord Kṛṣṇa is ten million times more dear to a devotee than life itself. And according to Śrīla Viśvanātha Cakravartī Ṭhākura, even by rendering devotional service for the total duration of one thousand universal creations, a devotee cannot repay the debt he feels to the Lord for having awarded him loving service to the Lord’s lotus feet. The Lord appears within the heart as the Supersoul and externally both as the spiritual master and as His literary incarnation — the supreme Vedic knowledge of Bhagavad-gītā and Śrīmad-Bhāgavatam.