Sermons found within the mūla-śāstras like Vedas, Purāṇas, Upaniṣads etc. and the writings of the pūrvācāryas of the Sampradāya, if, are found actually (not just superficially or apparently) contradicted by any later writing of that Sampradāya in an irresolvable way, it is proven that such a later writing is unauthentic due to being inflicted by the flaw of heresy with the predecessors. Hence, Lord Kṛṣṇa also stresses on the validity of the mūla-śāstras and the writings written on their basis in BG 16.23 & 24. The same has been again stressed in the Brahma Yāmala verse cited in Bhaktirasāmṛtasindhu i.e. “śruti-smṛti-purāṇādi….utpātāyaiva kalpate”. For the same reason, Śrīla Jīva Gosvāmī propounds in his Sarvasamvādinī commentary on Tattva Sandarbha that even if an incarnation of Lord Nārāyaṇa like Lord Buddha comes and preaches gospels contrary to the Vedic literature, such instructions given by the Lord Himself should be rejected by the steadfast followers of the Vedic religion. The is, indeed, a very heavy and noteworthy assertion made by Śrīla Jīva Gosvāmī. Evidence –
ya evānādi-siddhaḥ sarva-kāraṇasya bhagavato’nādi-siddhaṁ punaḥ punah sṛṣṭy-ādau tasmād evāvirbhūtam apauruṣeyam vākyam | tad eva bhramādi-rahitaṁ sambhāvitam | tac ca sarva-janakasya tasya ca sad-upadesāyāvaśyakaṁ mantavyam | tad eva cāvyabhicāri pramāṇam | tac ca tat-krpayaā ko’pi ko’pi gṛhṇāti | kutarka-karkaśā mūdhā vā tan na gṛhṇantu nāma, teṣām apramā-padaṁ katham apayātu ? na ceśvara-vihita-vaidyakādi-śāstram amatam | pramāṇābhāvād itaravad yātīti cen, na | pareṣāṁ tad-anugatatvād eva śāstratva-vyavahāraḥ | na ca buddhasyāpīśvaratve sati tad-vākyaṁ ca pramāṇaṁ syād iti vācyam | yena śāstrena tasya īśvaratvam manyāmahe, tenaiva tasya daitya-mohana-śāstra-karitvenoktatvāt |
“The Supreme Personality of Godhead is without beginning, perfect, and the cause of all causes, and from Him only emanate the perfect and super-human Vedic words which are without beginning again and again in the beginning of every creation. Only these words can possibly be devoid of the four defects. Since the Lord is the creator of all beings, His words must be considered necessary for the good instruction of all. Only this is an infallible means for acquiring knowledge, and it is accepted by those who have received His mercy. Let the fools and those whose hearts were hardened by bad logic not accept it. How can someone remove their ignorance? The other śāstras prescribed by God, such as the Āyurveda, etc. are not to be disregarded either. If someone objects that they lack authority just as other books, we say that it is not so, for they follow the Vedas and are thus commonly accepted as śāstra. Nor it can be said that Lord Buddha’s words are also authoritative because He is also an incarnation of God, for the same scripture by which we accept His divinity also states that He is the compiler of a scripture meant to deceive the demons.”
Hence, an explicit indication is given here that even if the Supreme Lord imparts instructions not in alignment with the Vedic literature, they must be discarded.
— Bhaktirasavedantapithadhisvara Acarya Sri RKDS ‘AV’ Gurupada