(37) The Buddha taught the Metta Sutta to deter deities from causing dreadful sights and terror
Reflecting as to which place would be most appropriate for these Bhikkhus, it was stated that the Buddha found no place within the whole region of Jambudipa suitable for the purpose of meditation other, than the place where they were residing. Therefore, the Buddha ordered them to return to the forest retreat from where they had come. The advice given by the Buddha was: "You should learn the Metta Sutta Paritta if you all wish to escape from the dangers brought about by the deities. This Metta Sutta would serve as a deterrent to the perils caused by those deities. It also help towards better realisation of the Dhamma in the practice of Kammatthana (meditation)." Furthermore, it has been elucidated in the Commentary as stated below :
Imanca suttam - This Metta Sutta also, masassa - during one month, atthasu dhammasavanadivasesu - for a period of eight days while occupied in preaching and listening to the sermon (it means waxing and waning days of the fifth, eighth, fourteenth, and fifteenth altogether eight days in a month), gandim akotetva - beat the wooden drum, ussaretha - utter incantations, dhammakatham karotha - go on preaching, sakacchatha - make deliberations, anumodatha - and this Paritta be expressed with anumodana - great rejoicing. Idameva kammatthanam - this metta meditational exercise, as Evama - be relied upon, bhavetha - be developed, bhahulikarotha - (and) be done or performed several times. Tepi amanussa - these Devas or deities also, Yo - will cause you, tam bherava-rammanam - no such dreadful sights and sensations, na dassanti - (and) will not make a display, nay, i.e. will not strike terror into your hearts. This advice was given by the Enlightened One in passing.
After taking instructions from the Buddha, the Bhikkhus returned to their former place in the forest, near their monastery. They stayed on in that place practising meditation and reciting the Metta Sutta according to the teachings of the Lord Buddha. These Devas or deities having felt that "these Noble Ones are sympathetic and are really bent upon bestowing upon us a lot of advantages by developing loving-kindness", were so pleased and happy that they even assisted the Bhikkhus in every way possible to fulfil their needs. Under these favourable circumstances, all these Bhikkhus after developing Metta and indulging in the practice of Vipassana based upon metta-jhana, were said to have attained Arahatship reaching the stage of arahattaphala during that period of Vassa. As such, at the commencement of the verse eulogising the virtues of this Metta Sutta, it has been mentioned that in view of the powerful influence of the noble attributes of this Metta-paritta, the Guardian. Devas of the trees had refrained themselves from displaying or exhibiting horrible sights and sensational objects to frighten the Bhikkhus.