The Isshi-koshosoku is a personal reply made to a disciple living in Kuroda. In contrast to the Ichimai-kishomon, it offers an emotional statement from Honen's personal experience on the efficacy of nembutsu practice. Along with the Ichimai-kishomon, it is used frequently in daily services given by Jodo Shu priests. (SHZ. 499-500)
While living in the age of the degenerate Dharma (mappo), we should not doubt gaining birth in the Pure Land even if we hardly practice. Either one recitation or ten recitations leads equally to birth. Shakyamuni Buddha proclaimed that we should not doubt that Amida Buddha will ever abandon us, even if we are a person filled with defilement. Although so much time has passed since then, we should not doubt these words. Even once the Dharma has become totally extinct, sentient beings can still attain birth - so why not now? We should not doubt birth even though we cannot eliminate our defilements. Master Shan-tao said that "I myself am a foolishly defiled being (bonpu)".
There are many pure lands in the ten directions, but the reason why we aspire for birth in the Western Pure Land of Amida is that even those who have committed the ten evil acts (ju-aku) and the five grievous sins (gogyakuzai) can be born there. There are various buddhas, but the reason why we call on Amida is that he will arrive to personally welcome us after even three or five recitations. There are various practices, but the reason why we select the nembutsu is that it is assured by Amida's original vow.
When we aspire to gain birth through Amida's original vow, we should never be in doubt about its accomplishment. To be embraced by the original vow depends on our deep faith. It is difficult to attain a human birth, yet we have done this. It is difficult to encounter the original vow, yet we have found it. It is difficult to cultivate the mind which aspires for birth in the Pure Land, yet we have realized this. It is difficult to depart this world of suffering, yet we will most surely do so. It is difficult to be born in the Pure Land, yet this is surely the most joyous accomplishment.
If those defiled people who have committed the ten evil acts (ju-aku) and the five grievous sins (gogyakuzai) have faith they will be born in the Pure Land [through reciting the nembutsu], we should think how much more so for those who have not committed even small crimes. All remaining defiled beings gain birth, so why not all the good beings? If we believe that one or ten recitations are not in vain, then so much more unceasing practice. If we can be born through one recitation, why not many?
Amida Buddha fulfilled his words that he would not attain enlightenment until all beings who aspired to be born in his land were born there. Therefore, he will indeed welcome at the moment of death those now living in other worlds. Shakyamuni Buddha imparted this great wisdom, "It is well accomplished! Follow my teachings and leave birth and death behind." The buddhas of the six directions proclaimed, "It is joyous! We bear witness that birth in the Pure Land is non-retrogressive." You should joyfully prostrate to the heavens above whenever you encounter Amida's original vow. Either walking, standing, sitting or lying down, you will be rewarded - this is the grace of all the buddhas. It is said [in Amida's eighteenth original vow] that we should aspire in all sincerity to chant Amida's name even ten times. It is written [in Shan tao's Wang-sheng-li-tsan (Hymns in Praise of Birth)] that we should believe with all our heart that birth is firmly secured.
Honen's Koshosoku (Honen no koshosoku) from the Honen Shonin gyojoezu, Scroll 21, section 15.