CC Madhya 12.194

tomāra siddhānta-saṅga kare yei jane
‘eka’ vastu vinā sei ‘dvitīya’ nāhi māne
Word for word: 
tomāra — Your; siddhānta-saṅga — acceptance of the conclusion; kare — does; yei jane — the person who; eka — one; vastu — substance; vinā — except; sei — such a person; dvitīya — a second thing; nāhi māne — does not accept.
Translation: 
“One who participates in Your impersonal monistic philosophy does not accept anything but the one Brahman.”
Purport: 

The impersonal monist does not believe that God is the only object of worship and that the living entities are His eternal servants. According to the monists, God and the devotee may be separate in the material state, but when they are spiritually situated, there is no difference between them. This is called advaita-siddhānta, the conclusion of the monists. Monists consider devotional service of the Lord to be material activity; therefore they consider such devotional activities to be the same as karma, or fruitive activity. This monistic mistake is a great stumbling block on the road to devotional service.

Actually this discussion between Advaita Ācārya and Nityānanda was a mock fight to serve as a great instruction for all devotees. Śrī Nityānanda Prabhu wanted to point out that Advaita Ācārya, a pure devotee, did not agree with the monistic conclusion. The conclusion of devotional service is:

vadanti tat tattva-vidastattvaṁ yaj jñānam advayam
brahmeti paramātmeti
bhagavān iti śabdyate

“Learned transcendentalists who know the Absolute Truth call this nondual substance Brahman, Paramātmā or Bhagavān.” (Bhāg. 1.2.11)

Absolute knowledge consists of Brahman, Paramātmā and Bhagavān. This conclusion is not the same as that of the monists. Śrīla Advaita Ācārya was given the title of ācārya because He spread the bhakti cult, not the philosophy of monism. The true conclusion of advaita-siddhānta, expressed at the very beginning of the Caitanya-caritāmṛta (Ādi 1.3), is not the same as the philosophy of the monists. Here advaita-siddhānta means advaya-jñāna, or oneness in variety. Actually Śrīla Nityānanda Prabhu was praising Śrīla Advaita Ācārya through friendly mock fighting. He was giving the Vaiṣṇava conclusion in terms of the Bhāgavatam’s conclusive words, vadanti tat tattva-vidaḥ. This is also the conclusion of a mantra in the Chāndogya Upaniṣad, ekam evādvitīyam.

A devotee knows that there is oneness in diversity. The mantras of the śāstras do not support the monistic conclusions of the impersonalists, nor does Vaiṣṇava philosophy accept impersonalism without variety. Brahman is the greatest, He who includes everything, and that is oneness. As Kṛṣṇa says in the Bhagavad-gītā (7.7), mattaḥ parataraṁ nānyat: there is no one superior to Kṛṣṇa Himself. He is the original substance because every category emanates from Him. Thus He is simultaneously one with and different from all other categories. The Lord is always engaged in a variety of spiritual activities, but the monist cannot understand spiritual variety. The conclusion is that although the powerful and the power are one and the same, within the energy of the powerful there are varieties. In those varieties there is a distinction between the different parts of one’s personal self, between types of the same category, and between types of different categories. In other words, there is always variety in the categories, which are understood as knowledge, the knower and the knowable. Due to the eternal existence of knowledge, the knower and the knowable, devotees everywhere know about the eternal existence of the form, name, qualities, pastimes and entourage of the Supreme Personality of Godhead. Devotees never agree with the monists’ preaching of oneness. Unless one adheres to the conceptions of the knower, the knowable and knowledge, there is no possibility of understanding spiritual variety, nor can one taste the transcendental bliss of spiritual variety.

The philosophy of monism is an adjustment of the Buddhist philosophy of voidism. In a mock fight with Śrī Advaita Ācārya, Śrī Nityānanda Prabhu was refuting this type of monistic philosophy. Vaiṣṇavas certainly accept Lord Śrī Kṛṣṇa as the ultimate “one,” and that which is without Kṛṣṇa is called māyā, or that which has no existence. External māyā is exhibited in two phases — jīva-māyā, the living entities, and guṇa-māyā, the material world. In the material world there is prakṛti (material nature) and pradhāna (the ingredients of material nature). However, for one who becomes Kṛṣṇa conscious, the distinction between material and spiritual varieties does not exist. An advanced devotee like Prahlāda Mahārāja sees everything as one — Kṛṣṇa. As stated in Śrīmad-Bhāgavatam (7.4.37), kṛṣṇa-graha-gṛhītātmā na veda jagad īdṛśam. One who is in full Kṛṣṇa consciousness does not distinguish between things material and spiritual; he takes everything to be related to Kṛṣṇa and therefore spiritual. By advaya-jñāna-darśana, Śrīla Advaita Ācārya has glorified pure devotional service. Śrīla Nityānanda Prabhu herein sarcastically condemns the philosophy of the impersonal monists and praises the correct nondual philosophy of Śrī Advaita Prabhu.