The teachings of Shinran were not his own. They belonged to a lineage of Pure Land teaching, going back to the beginning of Mahayana Buddhism:
The “Seven Patriarchs of Jōdo Shinshū” are seven Buddhist monks venerated in the development of Pure Land Buddhism as summarized in the Jōdo Shinshū hymn Shoshinge . Shinran quoted the writings and commentaries of the Patriarchs in his major work, the Kyogyoshinsho , to bolster his teachings.
The Seven Patriarchs, in chronological order, and their contributions are…
Jōdo Shinshū - Wikipedia
We should also keep in mind that the Pure Land sutras, according to traditional Mahayana interpretation, were spoken by Shakyamuni Buddha himself.
Shinran was a Tendai monk who, for twenty years, did everything he could to attain enlightenment by his own efforts.
He decided on Amida and the Nembutsu as the easy path to enlightenment, as taught by the Pure Land patriarchs, after realizing that his own efforts were ego-centered and futile.
As evidenced by such works as the Kyogyoshinsho, Shinran was one of the most well-read Buddhist scholars of his time.