1824 AD ordained
At the age of twenty he was ordained a proper Buddhist monk (ordination name Vajirayan; Pali Vajirañāṇo), the ceremony taking place in the temple of the Emerald Buddha which was the Royal Chapel.
The same year, his father died. By tradition, Mongkut should have been crowned the next king, but the nobility instead chose the older, more influential and experienced Prince Jessadabodindra (Nangklao), son of a royal concubine rather than a queen. Perceiving the throne was irredeemable and to avoid political intrigues, Mongkut retained his monastic status.
Vajirayan became one of the members of the royal family who devoted his life to religion. He travelled around the country as a monk and saw the relaxation of the rules of Pali Canon among the Siamese monks he met, which he considered inappropriate. In 1829, at Phetchaburi, he met a monk named Buddhawangso, who strictly followed the monastic rules of discipline, the vinaya. Vajirayan admired Buddhawangso for his obedience to the vinaya, and was inspired to pursue religious reforms.
In 1835 he began a reform movement reinforcing the vinaya law that evolved into the Dhammayuttika Nikaya, or Thammayut sect. A strong theme in Mongkut's movement was that, "…true Buddhism was supposed to refrain from worldly matters and confine itself to spiritual and moral affairs."
In 1836, Vajirayan arrived at Wat Bowonniwet in what is now Bangkok's central district, but was then the city proper, and became the wat's first abbot (เจ้าอาวาส). During this time, he pursued a Western education, studying Latin, English, and astronomy with missionaries and sailors. Vicar Pallegoix of the Roman Catholic Archdiocese of Bangkok lived nearby; the two became close friends, and Vajirayan invited Pallegoix to preach Christian sermons in the wat. Vajirayan admired Christian morals and achievements as presented by the vicar, but could make nothing of Christian doctrine. It was then he made the comment later attributed to him as king: "What you teach people to do is admirable, but what you teach them to believe is foolish.
