Honen Shonin said in An Outline of Nembutsu:

"It is even more unfortunate that there are many who insult and deride the exclusive practitioners of Nembutsu today. These unfortunate circumstances were foretold by incarnate
Buddhas and Bodhisattvas a long time ago. Accordingly, Bodhisattva Manjusri stated, 'Nonvirtuous beings in the future world will recite the Name of Amida Buddha in the West, become detached from the delusive worlds of saṃsara by relying on the Primal Vow, and be
born in the Pure Land through the depth of their sincere heart.'

In the Hymns of the Nien-fo Liturgy, Master Shan-tao thus stated:
'At the very moment when the World-Honored One [Buddha Shakyamuni] was coming to the end of His sermon, He entrusted to Sariputra with great care the Name of Amida Buddha[1].
Over time, after the death of Buddha Shakyamuni, the five defilements proliferated, and many people began to slander the teaching of Nembutsu. Monastics and lay people alike began to dislike the teaching of Nembutsu and ceased listening to it. This gave rise to the poison of anger, and when they saw Nembutsu practitioners, they struggled to incite intrigue and create grudges. Such people seem blind by nature and have no intrinsic goodness. Destroying the teaching designed for the instantaneous realization of Enlightenment, they will sink into the three lower realms for a long time. They will never be released from the three lower realms,
even though innumerable eons pass, more numerous than the number of particles released in the explosion of the earth.

The Sutra of Universal Enlightenment states: 'If a good man and woman listen to the teaching of Nembutsu, are overwhelmed both with sorrow at their state of ignorance and with unspeakable elation at the promise of birth in the Pure Land, and attempt to escape from the three lower realms, then it is clear that they have already cultivated the path leading to Enlightenment in a previous life[2]. Conversely, if a person does not rejoice even after listening to the teaching, it is a sign that he has just emerged for the first time from the three lower realms.'

Those who criticize and slander the teaching of Nembutsu are merely obsessed with illogical
reasons, which are not worthy of debate."[3]

Commentary:
“'Nonvirtuous beings in the future world” refer to us, ordinary people of blind passions living in the Last Dharma Age. We become assured of liberation from samsara (“become detached from the delusive worlds of saṃsara”)by saying the Nembutsu of faith in accordance with the Primal Vow of Amida Buddha.

There is a reason why this precious teaching was slandered in the past and is still slandered today. By being slandered today I refer to those belonging to other Paths[4]who look down on us for having a teaching too simple to be true, but also to false teachers who wear the robes and kesas of Nembutsu faith in various branches of Jodo Shinshu and who deny the existence of Amida Buddha by calling Him a myth, symbol and metaphor or who do not admit that the Pure Land is a real enlightened place to be attained after death. They too consider our devotional approach and simple faith to be ridiculous and dismiss it as folk religion.

Why there are so many wrong views about a Path like ours? Because it is the quickest way through which all beings (not only the spiritual elites) can attain Liberation. The Primal Vow is "the teaching designed for the instantaneous realization of Enlightenment" after death and rebirth in the Pure Land, so it’s too easy to be left without complications. Various Maras and evil spirits who don’t want people to escape their grasp, are even more decided to possess and influence those who can infiltrate and stop the spreading of the Primal Vow than those who teach self-power methods which are hard to follow anyway in this Last Dharma Age. However, the Path of faith in the real, living Amida Buddha will endure because it is the teaching in perfect correspondence with the times and the capacities of beings, thus making it the Way that all Buddhas support and protect.

Unfortunately, the karma of those who slander the Nembutsu teaching, by denying the very existence of Amida Buddha and His Pure Land is so evil that they will be reborn in the Avici hell where they stay for a time impossible to calculate with our limited minds. People like them “seem blind by nature and have no intrinsic goodness” which means that they do not have “good from the past”, as Rennyo Shonin explained. The good from the past represents our good karma from past lives and this life (prior to the awakening of faith) which manifests itself as an opening or receptive state of mind towards the message of the Primal Vow of Amida Buddha.

In deep harmony with His Master Honen, Shinran Shonin gave us an advice on how to treat those who slander the Nembutsu teaching both from the outside or inside of our school:

“Suppose that all other schools joined together in declaring, ‘The Nembutsu is for the sake of worthless people; that teaching is shallow and vulgar.’ Even then, without the slightest argument, one should reply ‘When foolish beings of inferior capacity like ourselves, persons ignorant of even a single letter, entrust themselves to the Vow, they are saved. Since we accept and entrust ourselves to this teaching, for us it is the supreme Dharma, though for those of superior capacity it might seem utterly base. Even though other teachings may be excellent, since they are beyond our capacity they are difficult for us to put into practice. The fundamental intent of the Buddhas is nothing but freedom from birth-and-death for all, ourselves and others included, so you should not obstruct our practice of the Nembutsu.’ If one responds without rancor thus, what person will do one harm? Moreover, there is an authoritative passage that states, ‘Where disputation takes place, blind passions arise. The wise keep their distance.’
Shakyamuni taught that there would be both people who entrust themselves to this teaching and people who abuse it. By the fact that I have entrusted myself fully to it and there are others who abuse it, I realize that the Buddha’s words are indeed true. Hence, I realize all the more clearly that my birth [in the Pure Land] is indeed firmly settled. If there were none who abused the teaching, then surely we would wonder why there are those who entrust but none who abuse it. This is not to say the Nembutsu necessarily must be slandered; I merely speak of the fact that the Buddha, knowing beforehand that there would be both those who trust and those who slander, taught this so people would have no doubts.”[5]

The words “where disputation takes place, blind passions arise. The wise keep their distance” are similar with Honen’s, “those who criticize and slander the teaching of Nembutsu are merely obsessed with illogical reasons, which are not worthy of debate."

Our Path is not based on the flawed logic of the unenlightened minds but on the direct revelations and instructions of Shakyamuni about Amida Buddha in the Larger Sutra. It is a teaching that must be accepted in faith as He clearly instructed:

“If there are sentient beings who have heard it, they will attain the stage of non-retrogression for realizing the highest Enlightenment. This is why you should single-heartedly accept in faith, uphold, and chant this sutra, and practice in accordance with its teachings.”

Insisting further on the importance of not having any doubt about the contents of this sutra, Shakyamuni said:

“‘I have expounded this teaching for the sake of sentient beings and enabled you to see Amitayus (Amida) and all in His land. Strive to do what you should. After I have passed into Nirvana, do not allow doubt to arise.”[6]

When having such a clear instruction from the World Honored One and the testimony of Ananda who literally saw Amida and His Pure Land, what is there to be debated?

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[1] I think that Master Shan-tao refers here to the Smaller Amida Sutra (Amidakyo) where Shakyamuni speaks to Shariputra alone.
[2] They have "good from the past" as Rennyo Shonin said.
[3] The Promise of Amida Buddha - Honen's Path to Bliss; English translation of the Genko edition of the works of Honen Shonin - Collected Teachings of Kurodani Shonin: The Japanese Anthology (Wago Toroku), translated by Joji Atone and Yoko Hayashi, Wisdom Publications, Boston, 2011, p.150-151
[4] Not all followers of other schools are like this, of course. Here I refer only to a category of infatuated and self-deluded people.
[5] Tannisho,The Collected Works of Shinran, Shin Buddhism Translation Series, Jodo Shinshu Hongwanji-ha, Kyoto, 1997, p.669
[6]The Three Pure Land Sutras - A Study and Translation from Chinese by Hisao Inagaki in collaboration with Harold Stewart, Bukkyo Dendo Kyokai and Numata Center for Buddhist Translation and Research, Kyoto, 2003, p.70