This sannyāsī was Śrīpāda Śaṅkarācārya. In order to overcome the effects of Buddhist philosophy and spread Vedānta philosophy, Śrīpāda Śaṅkarācārya had to make some compromise with the Buddhist philosophy, and as such he preached the philosophy of monism, for it was required at that time. Otherwise there was no need for his preaching Māyāvāda philosophy. At the present moment there is no need for Māyāvāda philosophy or Buddhist philosophy, and Lord Caitanya rejected both of them. This Kṛṣṇa consciousness movement is spreading the philosophy of Lord Caitanya and rejecting the philosophy of both classes of Māyāvādī. Strictly speaking, both Buddhist philosophy and Śaṅkara's philosophy are but different types of Māyāvāda dealing on the platform of material existence. Neither of these philosophies has spiritual significance. There is spiritual significance only after one accepts the philosophy of Bhagavad-gītā, which culminates in surrendering unto the Supreme Personality of Godhead. Generally people worship Lord Śiva for some material benefit, and although they cannot see him personally, they derive great material profit by worshiping him.
(SRIMAD BHAGAVATAM-------4:24:17--------PURPORT).

The conditioned soul is often frustrated in trying to understand the distinctions between the material body, the Supersoul and the individual soul. There are two types of Māyāvādī philosophers—the followers of the Buddhist philosophy and the followers of the Śaṅkara philosophy. The followers of Buddha do not recognize that there is anything beyond the body; the followers of Śaṅkara conclude that there is no separate existence of the Paramātmā, the Supersoul. The Śaṅkarites believe that the individual soul is identical with the Paramātmā in the ultimate analysis. But the Vaiṣṇava philosopher, who is perfect in knowledge, knows that the body is made of the external energy and that the Supersoul, the Paramātmā, the Supreme Personality of Godhead, is sitting with the individual soul and is distinct from him.
(SRIMAD BHAGAVATAM-------4:28:40--------PURPORT).

The most prominent Māyāvādī scholar, Sadānanda Yogīndra, has written a book called Vedānta-sāra, in which he expounds the philosophy of Śaṅkarācārya, and all the followers of Śaṅkara's philosophy attribute great importance to his statements. In this Vedānta-sāra Sadānanda Yogīndra defines Brahman as sac-cid-ānanda combined with knowledge and without duality, and he defines ignorance (jaḍa) as knowledge distinct from that of sat and asat. This is almost inconceivable, but it is a product of the three material qualities. Thus he considers anything other than pure knowledge to be material. The center of ignorance is considered to be sometimes all-pervading and sometimes individual. Thus according to his opinion both the all-pervading Viṣṇu and the individual living entities are products of ignorance.
(SRI CAITANYA CARITAMRTA------1:7:114-------PURPORT).

This verse indicates that oṁkāra, or praṇava, is a direct representation of the Supreme Personality of Godhead. Therefore if at the time of death one simply remembers oṁkāra, he remembers the Supreme Personality of Godhead and is therefore immediately transferred to the spiritual world. Oṁkāra is the basic principle of all Vedic mantras, for it is a representation of Lord Kṛṣṇa, understanding of whom is the ultimate goal of the Vedas, as stated in the Bhagavad-gītā (vedaiś ca sarvair aham eva vedyaḥ (B.G.15:15). Māyāvādī philosophers cannot understand these simple facts explained in the Bhagavad-gītā, and yet they are very proud of being Vedāntīs. Sometimes, therefore, we refer to the Vedāntī philosophers as Vidantīs, those who have no teeth (vi means "without," and dantī means "possessing teeth"). The statements of the Śaṅkara philosophy, which are the teeth of the Māyāvādī philosopher, are always broken by the strong arguments of Vaiṣṇava philosophers such as the great ācāryas, especially Rāmānujācārya. Śrīpāda Rāmānujācārya and Madhvācārya break the teeth of the Māyāvādī philosophers, who can therefore be called Vidantīs, "toothless."
(SRI CAITANYA CARITAMRTA------1:7:128-------PURPORT).

Madhvācārya's devotion to the Lord and his erudite scholarship became known throughout India. Consequently the owners of the Śṛṅgeri-maṭha, established by Śaṅkarācārya, became a little perturbed. At that time the followers of Śaṅkarācārya were afraid of Madhvācārya's rising power, and they began to tease Madhvācārya's disciples in many ways. There was even an attempt to prove that the disciplic succession of Madhvācārya was not in line with Vedic principles. A person named Puṇḍarīka Purī, a follower of the Māyāvādaphilosophy of Śaṅkarācārya, came before Madhvācārya to discuss the śāstras. It is said that all of Madhvācārya's books were taken away, but later they were found with the help of King Jayasiṁha, ruler of Kumla. In discussion, Puṇḍarīka Purī was defeated by Madhvācārya. A great personality named Trivikramācārya, who was a resident of Viṣṇumaṅgala, became Madhvācārya's disciple, and his son later became Nārāyaṇācārya, the composer of Śrī Madhva-vijaya. After the death of Trivikramācārya, the younger brother of Nārāyaṇācārya took sannyāsa and later became known as Viṣṇu Tīrtha.
(SRI CAITANYA CARITAMRTA------2:9:245-------PURPORT).