As indicated in the above quotations, shinjin is accepting Amida Buddha’s Primal Vow that guarantees birth in the Pure Land.

In Letter 13 of Fascle I, Master Rennyo wrote: What, then, do we mean by shinjin?

First of all, when we set aside all auxiliary/mixed practices and giving no thought to deities or other buddhas, take refuge with singleness of heart in Amida Tathagata, the Tathagata embraces us with his light, never to abandon us. This is precisely how the one thought-moment of faith is decisively established.

And again, in Letter 10, Fascle III, he wrote:

The right understanding of our school’s mind at ease is relying single-heartedly and steadfastly on Amida Tathagata without striving. We recognize how “marvelously mysterious” it is that, although wretched beings burdened with evil karma and blind passion, the working of Amida’s Vow—the strong cause (of our birth in the Pure Land)—is directed toward saving just such worthless beings. And when that single thought free of doubt becomes firm, Amida unfailingly sends forth his unhindered light and embraces us. Those who have experienced a decisive settling of shinjin in this way will all, each and every one, be born in the fulfilled land—ten people out of ten. What this means, in other words, is that “settled shinjin” based on “Buddha-centered power” is established within us.

Essentially, what Master Rennyo wrote is relying “singlemindedly and solely” (isshin ikko) on Amida Buddha. That is exactly what the Venerable Master Shinran taught—absolute conviction of birth in the Pure Land because of Amida Buddha’s Primal Vow. This is solely due to the Buddha’s vow power, and is expressed using phrases such as, “shinjin of ‘merit transference’ of the ‘power of the Primal Vow’” (honganriki eko no shinjin) and “shinjin given to us by the Tathagata” (nyorai yori tamawaritaru shinjin).