Master Seikaku made a very simple statement about accepting the Primal Vow in faith on the basis that Shakyamuni himself told us about it and the One who made this Vow:
Nevertheless, since what you heard was said by a person whom you cannot think would speak a mere fabrication, a hundred thousand people might tell you differently but you would not accept it. Rather, you deeply trust what you heard first. This is called trust.
Now, believing in what Sakyamuni taught, entrusting yourself to Amida's Vow, and being without any doubt should be like this.”
Also the Masters of our tradition who followed Shakyamuni, including Shan-tao, Honen and Shinran accepted and transmitted to us the truth of the Primal Vow: