This is the fruit of having gone through the exercise of defending my acceptance of the authenticity of the Mahayana. I doubt it will be of any use to anyone, though I wanted to share anyway, if for no other reason than to recommend adopting opposing views for the purpose of establishing conviction.
There exists the Stainless One, Guru Shakyamuni Buddha, worthy of every praise and admiration who, motivated by great compassion for those suffering in Samsara, taught the way to end Samsara according to the needs of those in attendance, based on his perfect knowledge of Samara's functioning, which arose from his having completely ended the causes of suffering for himself.
Is it not imperative that there exist other beings who, upon hearing Guru Shakyamuni Buddha's teaching, putting it into practice, and following it to its fruition, have themselves also attained the sorrowless state?
Is it not therefore likely that such similarly Stainless Ones, similarly worthy of every praise and admiration who, similarly motivated by great compassion for those suffering in Samsara, would similarly teach the way to end Samsara according to the needs of those in attendance, based on their similarly perfect knowledge of Samsara's functioning, which similarly arose from their having completely ended the causes of suffering for themselves?
Is it not therefore reasonable to view as authentic, all of the myriad teachings and commentaries of the lineages which accord with Guru Shakyamuni Buddha's teaching?
By thinking in this way, I gain conviction in the authenticity and efficacy of the Mahayana in general, and of devotion to the guru in particular.