SrI yAmunAcArya's work called AgamaprAmANya, SrI rAmAnujAcArya's SrI bhAshya (II.2.42), and SrI vedAnta- desika's pAncarAtra-rakshA are among the well-known works expounding the greatness of the pAncarAtra Agama. Of these, I have access only to the English translation of SrI rAmAnuja's SrI bhAshya, and so the material below is extracted from this source only.
SrI rAmAnuja mainly gives passages from the mahAbhArata to support the authenticity of the pAncarAtra doctrine. In mahAbhArata, SrI vyAsa has elaborately described the pAncarAtra system in the chapter called the moksha-dharma. He starts with the question 'If a householder, or a brahmachArin, or a hermit, or a mendicant wishes to achieve success, what deity should he worship?', then explains at great length the pAncarAtra system, and then says, "From the lengthy story of the mahAbhArata, consisting of one hundred thousand sloka-s, this body of doctrine (the pAncarAtra) has been extracted - like amr*ta from the medicinal herbs and AraNyaka from the veda-s". vyAsa also says in the mahAbhArata that this great upanishad - again referring to the pAncarAtra - is consistent with the four veda-s, and is in harmony with sAnkhya and yoga, the concentrated application of knowledge and works
"idam mahopanishadam caturveda samanvitam | sAnkhya yoga kr*tAntena pancarAtAnuSabditam || nArAyaNa mukhodgItam nArado'SrAvayat purA |
(MB, SAnti 348.62-3)
Note the words "nArAyaNa mukhodgItam" - The promulgator of the pAncarAtra is none other than nArAyaNa Himself according to vedavyAsa.
The mahAbhArata says of the pAncarAtra - "This is excellent, this is Brahman, this is supremely beneficial. Fully agreeing with the r*g, the yajus, the sAman, and the atahrva veda-s, this doctrine will be truly authoritative".
It is to be noted that the author of the mahAbhArata is the same vyAsa who composed the brahma sutra-s earlier. As was just noted, there is explicit and detailed support for the pAncarAtra system in the mahAbhArata, including statements that the system is consistent with the four veda-s, etc. However, SrI Sankara has interpreted some of the sutra-s in the brahma-sutra as being opposed to the pAnca-rAtra system (details are covered in the next part in this series, and I had also shared some of SrI sankara's views in a previous post). SrI rAmAnuja questions these interpretations based on the very strong statements made by SrI vyAsa in the mahAbhArata advocating the pAncarAtra doctrine as an upanishad that is consistent with all the veda-s. The mahAbhArata also says "Of the whole pAncarAtra, nArAyaNa Himself is the promulgator". Since the matter to be known from the veda is nArAyaNa, and since nArAyaNa Himself is the promulgator of the entire pAncarAtra, including the nature of nArAyaNa and how to worship him, it is only logical that in the pAncarAtra all other doctrines are comprised. For this reason, the mahAbhArata says "The sAnkhya, the yoga,the veda-s, and the AraNyaka, being members of one another, are called the pAncarAtra".
Some of the samhita-s elevate the pAncarAtra to a position above the veda-s. SrI-praSna-samhitA says
vedam ekAyanam nAma vedAnAm Sirasi sthitham | tadartham pAnca-rAtram mokshadam tatkriyAvaham || 2.38. vAmana samhitA takes the same position, based on the argument that it is only the pAnca-rAtra which has salvation as the only objective, whereas the veda-s promote the other three values also (dharma, artha, and kAma). The other distinguishing feature attributed to the ekAyana is that unlike the veda-s which can only be studied by those who are qualified to perform the rituals prescribed therein, the ekAyana is meant for all those who are eligible to realize final emancipation. The bhAgavata doctrine is distinguished by its insistence on bhakti which is absolute and exclusive (ekAnta), towards a concrete iconic manifestation of bhagavAn. The relation between a bhakta and the bhagavat is similar to the relation between the nara and nArAyaNa. The realization of the all-importance of bhagavat (or nArAyaNa) is covered by the sAnkhya, and the procedure by which the bhakta communicates with Bhagavat is covered by the yoga.