Śrīla Rāmānanda Rāya said to Śrīman Mahāprabhu:

nija gūḍha-kārya tomāra prema āsvādana, ānuṣaṅge premamaya kaile tribhubana – “Your own secret purpose was to taste prema yourself, but incidentally you also caused the three worlds to be filled with it.” (CC. M. 8.280)

kaichana rādhā-premā kaichana (mora) madhurimā, kaichana bhābe tiṅho bhora – “What is the nature of Rādhā’s love? What is the nature of my own sweetness? What is the nature of the state in which she is engrossed?”

In the mood of Rādhā, Śrī Caitanyadeva fulfilled these three desires in endless ways. Having personally tasted that truth, the desire arose within him to cause all of mankind to experience as Śrīmatī’s maidservants the sweetness of rādhā-prema.

He established and proclaimed that desire in this world through his own dear companion Śrīla Rūpa Gosvāmipāda.

In his commentary on this verse, Śrīla Viśvanātha Cakravartipāda has written:

śrī-caitanya-mahāprabhor mano’bhīṣṭaṁ mano’bhilaṣitaṁ śrīmad-bhagavad-bhakti-rasa-śāstraṁ bhūtale yena rūpeṇa sthāpitaṁ nirūpitam.

The meaning is that Śrī Caitanyadeva’s desire, namely, that scriptures describing bhagavad-bhakti-rasa be established within this world, was fulfilled by Śrī Rūpa.

Even though there were various bhakti-rasa-śāstras circulated prior to the Caitanya Era, Mahāprabhu’s specific contribution, the lofty ujjvala-rasamaya-bhakti (devotion pervaded by the sentiment of unrestrained love or passion) following the mood of the beautiful young girls of Vraja, was introduced and described by Śrīmad Rūpa Gosvāmipāda in Bhakti-Rasāmṛta-Sindhu, Ujjvala-Nīlamaṇi, Vidagdha-Mādhava-Nāṭaka, Lalita-Mādhava-Nāṭaka, Stava-Mālā and others.

That ujjvala-rasa of Vraja was previously beyond the reach of even Brahmā, Śiva and Uddhava, but by the mercy of Śrīman Mahāprabhu, the mantra-draṣṭā-ṛṣi (a sage who sees mantras) Śrīmad Rūpa Gosvāmipāda has composed many rasika books and dramas to enable even ordinary kali-yuga-jīvas to taste the nectar.
Other than Śrī Rūpa, no one was able to impart this subject matter.

The natural form of Śrī Bhagavān is rasa. If he hadn’t descended as the cloud of mercy Śrī Gaurāṅga to bless the world, the jīvas would never have been able to receive this rasa.
By the direct inspiration of Śrīman Mahāprabhu, Śrī Rūpa Gosvāmipāda has propagated the knowledge of rasa-tattva.

Śrī Rūpa personally mentioned this in the beginning of his Bhak-ti-Rasāmṛta-Sindhu:

hṛdi yasya preraṇayā pravartito’haṁ varāka-rūpo’pi |
tasya hareḥ pada-kamalaṁ vande caitanya-devasya ||

“I praise the lotus feet of Śrī Kṛṣṇa Caitanyadeva, by whom I have been inspired and engaged in the composition of this book about bhakti-rasa, though I am nothing more than an insignificant jīva.”

At Mahāprabhu’s urging, his dear companion Śrī Rūpa Gosvāmipāda has established the Lord’s beloved vraja-rasa within this world via the medium of books.

Therefore, those who are very eager to taste the sweet, most exalted transcendental bhāgavata-rasa of Vraja must take shelter at the feet of Śrīmad Rūpa Gosvāmipāda, the beloved associate of Śrī Gaurasundara, the giver of rasa and embodiment of blissful līlā, and make a serious study of the rasa-granthas he has composed.

– Srila Ananta das Babaji Maharaj, tika to Prema Bhakti Chandrika, verse 2