CC Madhya 18.115

yei mūḍha kahe, — jīva īśvara haya ‘sama’
seita ‘pāṣaṇḍī’ haya, daṇḍe tāre yama
yei mūḍha — any foolish person who; kahe — says; jīva — the living entity; īśvara — the supreme controller; haya — are; sama — equal; seita — he; pāṣaṇḍī haya — is a first-class atheist; daṇḍe — punishes; tāre — him; yama — the superintendent of death, Yamarāja.
“A foolish person who says that the Supreme Personality of Godhead is the same as the living entity is an atheist, and he becomes subject to punishment by the superintendent of death, Yamarāja.

Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura says that the word pāṣaṇḍī refers to one who considers the living entity under the control of the illusory energy to be equal with the Supreme Personality of Godhead, who is transcendental to all material qualities. Another kind of pāṣaṇḍī is one who does not believe in the spirit soul, the superior potency of the Lord, and therefore does not distinguish between spirit and matter. While describing one of the offenses against chanting the holy names, specifically the offense called śruti-śāstra-nindana (blaspheming the Vedic literature), Jīva Gosvāmī states in his Bhakti-sandarbha, yathā pāṣaṇda-mārgeṇa dattātreyarṣabha-devopāsakānāṁ pāṣaṇḍīnām. “Worshipers of impersonalists like Dattātreya are also pāṣaṇḍīs.” Concerning the offense of ahaṁ-mama-buddhi, or dehātma-buddhi (considering the body to be the self), Jīva Gosvāmī states, deva-draviṇādi-nimittaka-‘pāṣaṇḍa’-śabdena ca daśāparādhā eva lakṣyante, pāṣaṇḍa-mayatvāt teṣām: “Those who are overly absorbed in the conception of the body and the bodily necessities are also called pāṣaṇḍīs.” Elsewhere in the Bhakti-sandarbha it is stated:

uddiśya devatā evajuhoti ca dadāti ca
sa pāṣaṇḍīti vijñeyaḥ
svatantro vāpi karmasu

“A pāṣaṇḍī is one who considers the demigods and the Supreme Personality of Godhead to be one; therefore a pāṣaṇḍī worships any kind of demigod as the Supreme Personality of Godhead.” One who disobeys the orders of the spiritual master is also considered a pāṣaṇḍī. The word pāṣaṇḍī has been described in many places in Śrīmad-Bhāgavatam, including 4.2.28, 30 and 32, 5.6.9, and 12.2.13 and 3.43.

On the whole, a pāṣaṇḍī is a nondevotee who does not accept the Vedic conclusions. In the Hari-bhakti-vilāsa (1.117) there is a verse quoted from the Padma Purāṇa describing the pāṣaṇḍī. Śrī Caitanya Mahāprabhu quotes this verse as the following text.