Shinran on the Twentieth Vow wrote:
"Meditative or nonmeditative single mind" refers to seeking the power of the Primal Vow with a mind that believes in [the recompense of] evil and good. This is termed the single mind of self-power.
"Root of good" refers to the Tathagata's auspicious Name. This auspicious Name is perfectly possessed of the myriad goods; it is the root of all goods. Hence the term "root of good."
"Root of virtue" refers to the Tathagata's virtuous Name. This virtuous Name, with but a single utterance, fulfills the supreme virtues [in practicers], transforming all their evil. It is the source of the virtuous names [of the Buddhas] throughout the ten quarters in the past, present, and future. Hence it is termed the "root of virtue."
Sakyamuni Buddha revealed this store of virtue to encourage and guide beings of the defiled worlds of the ten quarters. Amida Tathagata, in the beginning, established the Vow that beings ultimately attain birth in order to compassionately lead the multitudinous ocean of all beings.
The compassionate Vow already exists. It is the "Vow of cultivating the root of virtue," also known as the "Vow of settlement of birth through placing thoughts [on the Pure Land]," the "Vow stating 'If they do not ultimately attain birth,'" and the "Vow of sincere mind and directing merit."
In the Larger Sutra, the Vow is stated:
If, when I attain Buddhahood, the sentient beings of the ten quarters, on hearing my Name, should place their thoughts on my land, cultivate the root of all virtues, and direct their merits with sincere mind desiring to be born in my land, and yet not ultimately attain it, may I not attain the supreme enlightenment.
Further, [the Larger Sutra] states:
They doubt these wisdoms and do not entrust themselves. And yet, believing in [the recompense of] evil and good, they aspire to be born in that land through cultivating the root of good. Such sentient beings will be born in the [womb] palace.
Further, it states:
People who lack the root of good
Are not able to hear this sutra,
But those who observe the precepts without fault
Are able to hear the right dharma.
The Sutra of the Tathagata of Immeasurable Life states:
If, when I become Buddha, all the sentient beings of the countless lands should hear my Name being expounded and, taking it as their own root of good, direct their merits toward the land of bliss, and yet not attain birth, may I not attain enlightenment.
The Sutra of the Enlightenment of Ultimate Equality states:
People not possessed of this virtue
Are unable to hear the name of this sutra;
Only those who have observed the precepts without fault
Have now come to hear the right dharma.
Those of evil, arrogance, the hindrance of passions, and indolence
Will have difficulty entrusting themselves to this dharma;
But those who have encountered Buddhas in previous lives
Will listen joyfully to the teaching of the World-honored one.
Rare is it to obtain human life,
And difficult to encounter a Buddha's appearance in the world;
Hard is it to attain the wisdom of entrusting:
Should you meet with and hear this teaching, pursue it with diligence.
The Contemplation Sutra states:
The Buddha said to Ananda, "You must hold firmly to these words. To hold to these words is to hold to the Name of the Buddha of immeasurable life."
The Amida Sutra states:
You cannot be born in that land through the cause of small roots of good and small virtues. Hearing Amida Buddha's Name being expounded, hold steadfast to it.
The Master of Kuang-ming temple states:
Although the many other practices are termed "good acts," when viewed in relation to the nembutsu, one finds them altogether beneath comparison. For this reason, many passages in the sutras extensively praise the efficacy of the nembutsu. For example, in such passages as the Forty-eight Vows in the Sutra of the Buddha of Immeasurable Life, it is clear that by singly practicing the saying of the Name alone, one attains birth.
Further, as taught in the Amida Sutra, by singly practicing the saying of Amida's Name for one to seven days, one attains birth.
Further, the witness of the Buddhas throughout the ten quarters, countless as the sands of the Ganges, is not hollow.
Again, the passages of this [Contemplation] Sutra presenting meditative and nonmeditative good acts show that by singly practicing the saying of the Name alone, one attains birth.
Such examples are not few. This completes the general exposition of nembutsu-samadhi.