Below is my sincere attempt in creating a “big picture” of Lord’s teachings to Sanatana Goswami.
In a nutshell, Lord Caitanya’s instructions to Sanatana Goswami covers the topics of God’s transcendental forms, His opulences, and devotional service.
Sanatana’s Goswami’s Surrenders to the Lord: In Chapter 20, Sanatana Goswami surrenders to Lord Caitanya. He falls at the Lord’s feet declaring himself lowborn, coming from bad association, most wretched, suffering in the dark well of material enjoyment, unaware about the goal of life, and foolishly thinks he is learned. He prays to the Lord to save him from material existence. He does not know who he is, what is duty is, why he suffers from material miseries, and how to be relieved from material entanglement. By this, Sanatana Goswami teaches us how to approach a spiritual master and inquire from him submissively (BG 4.34).
Jiva’s constitutional position: Out of His causeless mercy, Lord Caitanya instructs Sanatana Goswami about threefold material miseries (adhyatmika-klesa, adhibhautika-klesa, and adhidaivika-klesa). If one does not know why he is suffering from these three miseries, he must be considered ignorant. He should therefore approach a spiritual master. Lord Caitanya instructs Sanatana about one’s constitutional position – that one is a pure living soul and an eternal servant of Krsna. [CC Madhya 20.108]. By giving the analogy of sun and sunshine, the Lord explains the qualitative and quantitative aspects of the soul – the soul is transcendental, simultaneously one and different from Krsna. Though the living entity has the potency to understand the spiritual energy, he identifies with the body due to false ego. He suffers repeated birth and death in various species. The conditioning of the living entity is due to forgetfulness of his relationship with the Supreme Lord in the superior energy. Only when his original identity is revived does one become liberated. With the help of guru, sadhus and sastra , the living entity can disentangle himself from the material energy and re-establish his eternal relationship with Krishna.
Energies of the Lord: Lord Caitanya explains the material, spiritual and marginal energies of the Krsna. He is the source of the material world as well as the spiritual world. Krishna is ashraya-tattva (the shelter of everything). Everything else is ashrita-tattva. Everything is sheltered directly or indirectly by Krishna. He is the Absolute Truth and His form is eternal, full of knowledge, and full of bliss. [CC Madhya 10.108-151]
Aspects of the Absolute Truth: Lord Caitanya discusses the three aspects of the Absolute Truth, namely brahman , paramatma and bhagavan , and also gives a full description of the various Vishnu expansions. He elucidates the six varieties of avataras (purusa , lila , guna , manvantara , yuga and saktyavesa ) and establishes Lord Krishna as svayam-bhagavan , the original source of everything. Krishna performs His eternal pastimes in multiple universes simultaneously, although His most confidential and intimate pastimes are exclusively enacted in Shri Vrindavana dham. The transcendental form of the Lord can only be understood through the process of devotional service in full Krishna consciousness. [CC Madhya 20.136-164]
Spiritual and Material Universes: Lord Caitanya describes the different spiritual worlds. He describes Devi dham, Mahesha dham, Hari dham and Goloka dham (Gokula, Mathura, Dvaraka). He extensively describes the qualities and opulence of Krsna which enchants everyone in these universes. The spiritual world, which lies beyond the Viraja river, comprises 3/4 of the total energies of the Supreme Lord. In the Lord’s supreme planet, Goloka, all the opulences of the other spiritual planets are displayed. The full extent of Krsna’s greatness is inconceivable, even to Brahma. Though Krsna exhibits unlimited wonderful pastimes in Vaikuntha as Vasudeva and Sankarshana, and in the material world in so many incarnations, His most important activities are His pastimes in human form, which include His pastimes in Vrindavana, Mathura and Dwarka. [CC Madhya 21.1-106] “Lord Kṛṣṇa has many pastimes, of which His pastimes as a human being are the best. His form as a human being is the supreme transcendental form. In this form He is a cowherd boy. He carries a flute in His hand, and His youth is new. He is also an expert dancer. All this is just suitable for His pastimes as a human being. ”
The Opulence and Sweetness of Lord Krsna: Lord Caitanya describes the beauty of Lord Krsna. He instructs Sanatana Goswami that even a fractional understanding of Kṛṣṇa’s beauty can merge all three worlds in the ocean of love. He attracts all living entities within the three worlds. The transcendental form of Kṛṣṇa is shown to the world by Lord Kṛṣṇa’s internal, spiritual energy, which is a transformation of pure goodness. This jewellike form is the most confidential treasure of the devotees. This form is manifested from Kṛṣṇa’s eternal pastimes. The wonderful form of Kṛṣṇa in His personal feature is so great that it attracts even Kṛṣṇa to taste His own association. Indeed, Kṛṣṇa becomes very eager to taste it. Total beauty, knowledge, wealth, strength, fame and renunciation are the six opulences of Kṛṣṇa. He is eternally situated in His opulences. The Lord explained the beauty of Krishna’s ornaments, the attractive power of His flute, and the absorption of the gopis in Krishna’s beauty. [CC Madhya 21.107-149]
The Process of Devotional Service: In Chapter 22, Lord Caitanya introduces the process of devotional service. Krsna can be approached only by the process of devotional service. Indeed, it is the conclusion of all scriptures that Krsna is the only object of worship. Though fruitive activities, mystic yoga, and the speculative search for knowledge are sometimes praised in the Vedic literatures, the path of devotional service is the foremost. Karma, jnana, and yoga are indirect methods that cannot lead to the highest perfection of self-realization. All paths to self-realization ultimately depend on devotional service. [CC Madhya 22.1-18]
Everything else is subordinate to devotional service: Lord Caitanya then explains that any process short of devotional service is insufficient to free the conditioned soul from matter. Even if one executes his duties according to varnashrama principles, he will go to hell if he lacks Krsna consciousness. Therefore, surrender to Krsna makes the difference between bondage and liberation. Krsna is like the sun, maya like darkness. One who is illuminated by Krsna consciousness cannot be in maya. If the conditioned soul engages in the service of the Lord and simultaneously carries out the orders of his spiritual master and serves him, he can get out of the clutches of maya and become eligible for shelter at Kṛṣṇa’s lotus feet.
Association of Devotees: Good fortune begins when one gets association with pure devotees. It is neither possible to get liberated nor develop love of Godhead without the favor of a pure devotee. Firm conviction in the Lord’s order is faith which is favorable to the discharge of devotional service. Such faith is demonstrated in the conviction that by only serving Krsna, all other duties are fulfilled. [CC Madhya 22.19-62]
Three classes of devotees: Lord Caitanya explains the differences between the three classes of devotees. A faithful devotee is a truly eligible candidate for the loving service of the Lord. According to one’s faith, one is classified as a topmost devotee, an intermediate devotee or an inferior devotee. A devotee is considered superlative or superior according to his attachment and love. In one who has unflinching devotional faith in Kṛṣṇa, all the good qualities of Kṛṣṇa and the demigods are consistently manifest. However, he who has no devotion to the Supreme Personality of Godhead has no good qualifications because he is engaged by mental concoction in material existence, which is the external feature of the Lord. [CC Madhya 22.63-76]
Qualities of devotees: Lord Caitanya describes the 26 qualities of a devotee, and the behavior of a pure devotee. A devotee’s most formidable enemy is association with women in an enjoying spirit. Association with non-devotees is also condemned because it is also a formidable enemy on the path of devotional service. One has to fully surrender unto the lotus feet of Kṛṣṇa and give up attraction for women and non-devotees.
Six symptoms of surrender: Next the Lord explains the six divisions of surrender to Krishna. The six divisions of surrender are the acceptance of those things favorable to devotional service, the rejection of unfavorable things, the conviction that Kṛṣṇa will give protection, the acceptance of the Lord as one’s guardian or master, full self-surrender, and humility. When a devotee thus fully surrenders unto Kṛṣṇa’s lotus feet, Kṛṣṇa accepts him as one of His confidential associates. [CC Madhya 22.100-102]
Regulated & Spontaneous Devotional Service: Lord Caitanya defines the stages of vaidhi-bhakti (regulative devotional service) and raganuga-bhakti (spontaneous devotional service). Expanding on regulative devotion, LordCaitanya delineates the 64 items of devotional service that awaken one’s dormant consciousness of Krishna. He then discusses spontaneous devotional service. The primary characteristic of spontaneous love is deep attachment for the Supreme Personality of Godhead. Absorption in thought of Him is a marginal characteristic. Devotional service in spontaneous love is vividly expressed and manifested by the inhabitants of Vṛndavana. Devotional service that accords with their devotional service is called raganuga bhakti, or devotional service following in the wake of spontaneous loving service. [CC Madhya 22]
All glories to Srila Prabhupada!