As stated here, the Venerable Master abandoned the “miscellaneous practices” (the religious practices prescribed in the Tendai school) and relied on the “Primal Vow” (Amida Buddha’s vow to save all sentient beings) during the Shinyu year of Kennin, which is the 1st year of the Kennin era, or 1201 2 AD. He was then 29 years of age. The Venerable Master clearly states that he abandoned the “way of becoming enlightened through my own efforts,” and entered into the “way of becoming enlightened through reliance on Amida Buddha’s power to save” (tariki no michi).
In Letter Three of Eshinni-ko’s (the Venerable Master Shinran’s wife) letters, she refers to this event in the following words:
(The Venerable Master Shinran) descended the mountain (Mt. Hiei) and retreated to Rokkaku-do for a hundred days to pray for salvation. In a dream at dawn of the 95th day, Shotoku Taishirevealed a verse indicating the path to take. (The Venerable Master Shinran) immediately left Rokkaku-do and called on Master Honen to be shown the way. And just as he had confined himself for a hundred days at Rokkaku-do, he visited Master Honen for a hundred consecutive days, whether it rained or shined, regardless of the obstacles. From that “good person” he learned that only the Nembutsu is necessary to overcome life and death.
We do not know exactly what Shotoku Taishi’s words in the Venerable Master’s dream were. They should have been included in Eshinni-ko’s letters, but they are lost.
Regarding this, there is a poem titled Byokutsu-ge that is carved in stone in Shotoku Taishi’s mausoleum. In it, Shotoku Taishi states that he is the transformed body of Kuze Kannon, his wife the transformed body of Daiseishi, his mother the transformed body of Amida, and that they appeared in this world to save all sentient beings.
There are other indications pointing to the Venerable Master being lead to Master Honen by Shotoku Taishi, but they cannot be used as historical evidence. At any rate, the Venerable Master seems to have decided to visit Master Honen on the 95th day of his seclusion in Rokkaku-do. That was when he abandoned the way of trying to become enlightened through his own efforts and entered the way of relying on Amida Buddha’s Primal Vow.
As indicated in Eshinni-ko’s letter already quoted, the Venerable Master studied with Master Honen for a hundred consecutive days, earnestly seeking the way that would allow him to leave the world of delusion and suffering, and enter the world of enlightenment.
The Venerable Master expressed his deep emotions of that time in Koso Wasan (Japanese Poems on the Eminent Monks):
After long kalpas and many births,
We still did not know
The powerful conditions for release.
If Genka (Master Honen) had not appeared,
This life would also have passed in vain.
I was born and died from endless past, the Venerable Master says, and had been mired in this world of delusion, unable to know the power of Amida Buddha’s Primal Vow that allows me to leave this deluded world. If Master Honen had not lived and if I had not met him, I would have continued drifting aimlessly, never to be born in the Pure Land of Enlightenment.
There are many differences of opinion regarding when the Venerable Master turned to the Primal Vow (the 18th Vow), that is, when he received shinjin (the “faith” mind, or “true” mind). I believe it was at the age of 29, when he met Master Honen. Indications after that age can be considered a deepening of his thought, but I believe absolutely nothing could have swayed his conviction and peace of mind regarding his birth in the Pure Land that he received then.
At any rate, when the Venerable Master was 29 years of age, he became Master Honen’s disciple and began relying on Amida Buddha’s vow power to cause his birth in the Pure Land. That was when he changed his name to shuk’ka.