It is Buddhist tradition to believe the Truth and not who said it.
Supposing Nagarguna had established a religion
going further than the Buddha's preaching in the [i]Hinayana[/h]
(though now only the Theravadins of the Southern Buddhist traditions remain as an independent school)
then we should believe Nagarjuna and not the Buddha, since the former taught the ultimate and complete truth
In the Prajnaparamita Sutra
Our faith relies on truth and not on persons. We believe in the truth itself but not in letter or word of scripture. And in the ultimate but not in the incomplete truth.
Finally we lay stress on wisdom of realisation(prajna) and not on mere consciousness(vijnana) of the human mind.
In the Mahayana it is never said that the [edit: Theravada] is not Buddha-word,...
It is said that the Buddha preached the Lesser as a foundation for the Greater Vehicle
and this despite the fact that the Mahayana is already so complete
SN 20.7: Ani Sutta http://www.accesstoinsight.org/tipit....007.than.html Staying at Savatthi. "Monks, there once was a time when the Dasarahas had a large drum called 'Summoner.' Whenever Summoner was split, the Dasarahas inserted another peg in it, until the time came when Summoner's original wooden body had disappeared and only a conglomeration of pegs remained. "In the same way, in the course of the future there will be monks who won't listen when discourses that are words of the Tathagata — deep, deep in their meaning, transcendent, connected with emptiness — are being recited. They won't lend ear, won't set their hearts on knowing them, won't regard these teachings as worth grasping or mastering. But they will listen when discourses that are literary works — the works of poets, elegant in sound, elegant in rhetoric, the work of outsiders, words of disciples — are recited. They will lend ear and set their hearts on knowing them. They will regard these teachings as worth grasping & mastering. "In this way the disappearance of the discourses that are words of the Tathagata — deep, deep in their meaning, transcendent, connected with emptiness — will come about. "Thus you should train yourselves: 'We will listen when discourses that are words of the Tathagata — deep, deep in their meaning, transcendent, connected with emptiness — are being recited. We will lend ear, will set our hearts on knowing them, will regard these teachings as worth grasping & mastering.' That's how you should train yourselves."