Lord Śrī Caitanya Mahāprabhu bestows His causeless mercy upon His devotees in three features: His own direct appearance [sākṣāt], His prowess within someone He empowers [āveśa], and His manifestation [āvirbhāva].
PURPORT
The sākṣāt feature of Śrī Caitanya Mahāprabhu is His personal presence. Āveśa refers to invested power, like that invested in Nakula Brahmacārī. Āvirbhāva is a manifestation of the Lord that appears even though He is personally not present. For example, Śrī Śacīmātā offered food at home to Śrī Caitanya Mahāprabhu although He was far away in Jagannātha Purī, and when she opened her eyes after offering the food she saw that it had actually been eaten by Śrī Caitanya Mahāprabhu. Similarly, when Śrīvāsa Ṭhākura performed saṅkīrtana, everyone felt the presence of Śrī Caitanya Mahāprabhu, even in His absence. This is another example of āvirbhāva.
(CC Adi 10.56)
The appearance of Lord Śrī Caitanya Mahāprabhu in every devotee’s presence is called sākṣāt. His appearance in Nakula Brahmacārī as a symptom of special prowess is an example of āveśa.
(CC Adi 10.57)
In the Gaura-gaṇoddeśa-dīpikā (74) it is said that Nakula Brahmacārī displayed the prowess (āveśa) and Pradyumna Brahmacārī the appearance (āvirbhāva) of Śrī Caitanya Mahāprabhu. There are many hundreds and thousands of devotees of Lord Caitanya among whom there are no special symptoms, but when a devotee of Lord Śrī Caitanya Mahāprabhu functions with specific prowess, he displays the feature called āveśa. Śrī Caitanya Mahāprabhu personally spread the saṅkīrtana movement, and He advised all the inhabitants of Bhāratavarṣa to take up His cult and preach it all over the world. The visible bodily symptoms of devotees who follow such instructions are called āveśa. Śrīla Śivānanda Sena observed such āveśa symptoms in Nakula Brahmacārī, who displayed symptoms exactly like those of Śrī Caitanya Mahāprabhu. The Caitanya-caritāmṛta states that in the Age of Kali the only spiritual function is to broadcast the holy name of the Lord, but this function can be performed only by one who is actually empowered by Lord Kṛṣṇa. The process by which a devotee is thus empowered is called āveśa, or sometimes it is called śakty-āveśa. Pradyumna Brahmacārī was formerly a resident of a village known as Piyārīgañja in Kālnā. There is a description of him in the Antya-līlā of Śrī Caitanya-caritāmṛta, Second Chapter, and in the Antya-līlā of Śrī Caitanya-bhāgavata, chapters Three and Nine.
(CC Adi 10.59 purport)
In Āmbuyā-muluka there was a person named Nakula Brahmacārī, who was a perfectly pure devotee, greatly advanced in devotional service.
PURPORT
Śrīla Bhaktivinoda Ṭhākura says that Āmbuyā-muluka is the present Ambikā, a city in the Vardhamāna district of West Bengal. Formerly, during the Muslim regime, it was known as Āmbuyā-muluka. In this city there is a neighborhood called Pyārīgañja, and that is where Nakula Brahmacārī used to live.
(CC Antya 2.16)
Desiring to deliver all the people of Bengal, Śrī Caitanya Mahāprabhu entered the heart of Nakula Brahmacārī. Nakula Brahmacārī became exactly like a man haunted by a ghost. Thus he sometimes laughed, sometimes cried, sometimes danced and sometimes chanted like a madman. He continuously exhibited bodily transformations of transcendental love. Thus he cried, trembled, became stunned, perspired, danced in love of Godhead and made sounds like those of a cloud. His body shone with the same luster as that of Śrī Caitanya Mahāprabhu, and he showed the same absorption in ecstatic love of Godhead. People came from all provinces of Bengal to see these symptoms. He advised whomever he met to chant the holy names Hare Kṛṣṇa. Thus upon seeing him, people were overwhelmed with love of Godhead. When Śivānanda Sena heard that Śrī Caitanya Mahāprabhu had entered the body of Nakula Brahmacārī, he went there with doubts in his mind. Desiring to test the authenticity of Nakula Brahmacārī, he stayed outside, thinking as follows. “If Nakula Brahmacārī personally calls me and knows my worshipable mantra, then I shall understand that he is inspired by the presence of Śrī Caitanya Mahāprabhu.” Thinking in this way, he stayed some distance apart. There was a large crowd of people, some coming and some going. Indeed, some people in that great crowd could not even see Nakula Brahmacārī. In his inspired state, Nakula Brahmacārī said, “Śivānanda Sena is staying some distance away. Two or four of you go call him.” Thus people began running here and there, calling in all directions, “Śivānanda! Whoever is Śivānanda, please come. Nakula Brahmacārī is calling you.” Hearing these calls, Śivānanda Sena quickly went there, offered obeisances to Nakula Brahmacārī, and sat down near him. Nakula Brahmacārī said, “I know that you are doubtful. Now please hear this evidence with great attention.
(CC Antya 2.17 – 2.30)
“You are chanting the Gaura-gopāla mantra, composed of four syllables. Now please give up the doubts that have resided within you.”
PURPORT
Śrīla Bhaktivinoda Ṭhākura explains the Gaura-gopāla mantra in his Amṛta-pravāha-bhāṣya. Worshipers of Śrī Gaurasundara accept the four syllables gau-ra-aṅ-ga as the Gaura mantra, but pure worshipers of Rādhā and Kṛṣṇa accept the four syllables rā-dhā kṛṣ-ṇa as the Gaura-gopāla mantra. However, Vaiṣṇavas consider Śrī Caitanya Mahāprabhu nondifferent from Rādhā-Kṛṣṇa (śrī-kṛṣṇa-caitanya rādhā-kṛṣṇa nahe anya). Therefore one who chants the mantra “gaurāṅga” and one who chants the names of Rādhā and Kṛṣṇa are on the same level.
(CC Antya 2.31)
Śivānanda Sena thereupon developed full confidence in his mind that Nakula Brahmacārī was filled with the presence of Śrī Caitanya Mahāprabhu. Śivānanda Sena then offered him respect and devotional service. In this way, one should understand the inconceivable potencies of Śrī Caitanya Mahāprabhu. Now please hear how His appearance [āvirbhāva] takes place.
(CC Antya 2.32 – 2.33)
Śrī Caitanya Mahāprabhu delivered almost all the fallen souls by directly meeting them. He delivered others by entering the bodies of great devotees, such as Nakula Brahmacārī. And He delivered still others by appearing before them, as in the case of Nṛsiṁhānanda Brahmacārī. “I shall deliver the fallen souls.” This statement characterizes the Supreme Personality of Godhead.
PURPORT
The Lord always manifested His āvirbhāva appearance in the following four places: (1) the house of Śrīmatī Śacīmātā, (2) wherever Nityānanda Prabhu danced in ecstasy, (3) the house of Śrīvāsa (when kīrtana was performed), and (4) the house of Rāghava Paṇḍita. Lord Caitanya Himself appeared in these four places. (In this connection, one may consult text 34.)
(CC Antya 2.5-6)