What you inquire about in your letter is a passage from a sutra that states: “Those who attain shinjin and joy are equal to Tathagatas.” This is from the Garland Sutra and means that the person who rejoices in shinjin is the equal of all the Tathagatas. This is also indicated in Sakyamuni’s statement about those who realize shinjin and greatly rejoice: “The one who sees, reveres, and attains [the dharma] and greatly rejoices – that person is my excellent, close companion.”
Further, Amida’s Seventeenth Vow declares that the Buddha will not enter into perfect enlightenment if those who say the Name are not praised by all the countless Buddhas throughout the worlds in the ten quarters. The passage declaring the fulfillment of the Vow states: “Such people are praised by all the Buddhas and rejoice.”
You should have no doubts whatever concerning this matter.
Here I have recorded the passages on being equal to the Tathagatas.
Shoka 1 [1257], Tenth month, 10th day
Shinran
To Shinbutsu-bo