Sometimes it is necessary to preach a philosophical doctrine which is against the Vedic conclusion. In the Śiva Purāṇa it is stated that Lord Śiva said to Pārvatī that in the Kali-yuga, in the body of a brāhmaṇa, he would preach the Māyāvāda philosophy. Thus it is generally found that the worshipers of Lord Śiva are Māyāvādī followers. Lord Śiva himself says, māyāvādam asac-chāstram. Asat-śāstra, as explained here, means the doctrine of Māyāvāda impersonalism, or becoming one with the Supreme. Bhṛgu Muni cursed that persons who worshiped Lord Śiva would become followers of this Māyāvāda asat-śāstra, which attempts to establish that the Supreme Personality of Godhead is impersonal. Besides that, among the worshipers of Lord Śiva there is a section who live a devilish life. Śrīmad-Bhāgavatam and Nārada-pañcarātra are authorized scriptures that are considered sat-śāstra, or scriptures which lead one to the path of God realization. Asat-śāstras are just the opposite.
(Srimad Bhahavatam----4:2:28----purport).

Lord Śiva is always in meditation on the lotus feet of Vāsudeva, Śrī Kṛṣṇa. Thus the auspicious position of Lord Śiva is realized if one takes to the worship of Viṣṇu, because Lord Śiva says in the Śiva Purāṇa that the topmost worship is worship of Lord Viṣṇu. Lord Śiva is worshiped because he is the greatest devotee of Lord Viṣṇu. One should not, however, make the mistake of considering Lord Śiva and Lord Viṣṇu to be on the same level. That is also an atheistic idea. It is also enjoined in the Vaiṣṇavīya Purāṇa that Viṣṇu, or Nārāyaṇa, is the exalted Supreme Personality of Godhead, and no one should be compared to Him as equal, even Lord Śiva or Lord Brahmā, not to speak of other demigods.
(Srimad Bhahavatam----4:4:14----purport).

In the execution of devotional service, śravaṇaṁ kīrtanaṁ viṣṇoḥ (S.B.7:5:23) is especially stressed. This means that bhakti, or devotional service, begins by hearing and chanting about Viṣṇu. When we speak of Viṣṇu, we also refer to that which relates to Viṣṇu. In the Śiva Purāṇa, Lord Śiva recommends Viṣṇu worship to be the topmost worship, and better than Viṣṇu worship is worship of the Vaiṣṇava or anything that is related to Viṣṇu. The fact is explained herein that hearing and chanting about a Vaiṣṇava is as good as hearing and chanting about Viṣṇu, for Maitreya has explained that anyone who hears about Pṛthu Mahārāja with attention also attains the planet which Mahārāja Pṛthu attained. There is no duality between Viṣṇu and the Vaiṣṇava, and this is called advaya-jñāna.
(Srimad Bhahavatam----4:23:31----purport).

"The Māyāvāda philosophy," Lord Śiva informed his wife Pārvatī, "is impious (asac chāstra). It is covered Buddhism. My dear Pārvatī, in Kali-yuga I assume the form of a brāhmaṇa and teach this imagined Māyāvāda philosophy. In order to cheat the atheists, I describe the Supreme Personality of Godhead to be without form and without qualities. Similarly, in explaining Vedānta I describe the same Māyāvāda philosophy in order to mislead the entire population toward atheism by denying the personal form of the Lord." In the Śiva Purāṇa the Supreme Personality of Godhead told Lord Śiva:
dvāparādau yuge bhūtvā kalayā mānuṣādiṣu
svāgamaiḥ kalpitais tvaṁ ca janān mad-vimukhān kuru
"In Kali-yuga, mislead the people in general by propounding imaginary meanings for the Vedas to bewilder them." These are the descriptions of the Purāṇas.
(Sri Caitanya Caritamrta----1:7:110----purport).