The historical background which led Buddha to expound the Karaniya Metta Sutta is explained in the commentary written by Acariya Buddhaghosa, who received it from an unbroken line of Elders going back to the days of the Buddha himself.

It is told that the five hundred monks received instructions from the Buddha in the particular techniques of meditation suitable to their individual temperaments. They then went to the foothills of the Himalayas to spend the four months of the rains’ retreat by living a life of withdrawal and intensive medications. In those days, a month or two before the rains’ retreat started, monks from all parts of the country would assemble wherever the Buddha lived in order to receive direct instruction from the Supreme Master. Then they would go back to their monasteries, forest dwellings or hermitages to make a vigorous attempt at spiritual liberation. This was how these five hundred monks went to the Buddha, who was staying at Savatthi in Jeta’s Grove in the monastery built by Anathapondika.

After receiving instructions, they went in search of a suitable place, and during their wandering, they found a beautiful hillock at the foothills of the Himalayas. This, according to the commentary, “appeared like a glittering blue quartz crystal. It was embellished with a cool, dense, green forest grove and a stretch of ground strewn with sand, resembling a pearl net or a silver sheet, and was furnished with a clean spring of cool waters.” The bhikkhus were captivated by the sight. There were a few villages nearby, and a small market town ideal as an alms resort. The monks spent a night in the idyllic grove and the next morning went to the market town for alms.

The residents were overjoyed to see the monks since rarely did a community of monks come to spend the retreat in that part of the Himalayas. These pious devotees fed the monks and begged them to stay on as their guests, promising to build each a hut near the grove on the sandy stretch so that they could spend their days and nights plunged in meditation under the ancient boughs of the majestic trees. The bhikkhus agreed and the devotees of the area soon built little huts in the fridge of the forest and provided each hut with a wooden cot, a stool and pots of water for drinking and washing.

After the monks have settled down contentedly in these huts, each one selected a tree to meditate under, by day and by night. Now it is said that these great trees were inhabited by tree deities who had a celestial mansion built. These deities, out of reverence for the meditation monks, stood aside with their families. Virtue was revered by all, particularly so by deities, and when the monks sat under the trees, the deities, who were householders, did not like to remain above them. The deities had thought that the monks would remain only for a night or two, and gladly bore the inconvenience. But when day after day passed and the monks kept occupying the bases of the trees, the deities wondered when they would go away. They were dispossessed villagers whose houses had been commandeered by the officials of visiting royalty and they kept watching anxiously from a distance, wondering when they would get their houses back.

These dispossessed deities discussed the situation among themselves and decided to frighten the monks away by showing them terrifying objects, making dreadful noises, and creating a sickening stench. Accordingly, they materialised all these terrifying conditions and afflicted the monks. The monks soon grew pale and could not concentrate on their subjects of meditation. As the deities continued to harass them, they lost their basic mindfulness, and their minds seemed to become smothered by the oppressing visions, noise and stench. When the monks assembled to wait upon the most senior elder of the group, each one recounted his experiences. The Elder suggested, “Let us go, brethren, to the Blessed One and place our problem before him. There are two kinds of rains’ retreats — the early and the late. Though we will be breaking the early one by leaving this place, we can always take upon ourselves the late one after meeting the Lord.“ The monks agreed and they set out at once, it is said, without even informing the devotees.

By stages, they arrived at Savatthi, went to the Blessed One, prostrated at his feet, and related their frightful experiences, pathetically requesting another place. The Buddha, through his supernormal power, scanned the whole of India, but finding no place except the same spot where they could achieve spiritual liberation, told them, “Monks, go back to the same spot! It is only by striving these that you will effect the destruction of inner taints. Fear not! If you want to be free from the harassment caused by the deities, learn this sutta. It will be a theme for meditation as well as a formula for protection (paritta).”

Then the Master recited the Karaniya Metta Sutta — the Hymn of Universal Love — which the monks learned by rote in the presence of the Lord. Then they went back to the same place.

As the monks neared their forest, dwellings reciting the Metta Sutta, thinking and meditating on the underlying meaning, the hearts of the deities became so charged with warm feelings of good-will that they materialised themselves in human form and received the monks with great piety. They took their bowls, conducted them to their rooms, caused water and food to be supplied, and then, resuming their normal form, invited them to occupy the bases of the trees and meditate without any hesitation or fear.

Further, during the three months of the rains’ residence, the deities not only looked after the monks in every way but made sure that the place was completely free from any noise. Enjoying perfect silence, by the end of the rainy season, all the monks attained the pinnacle of spiritual perfection. Every one of the five hundred monks had become an arahant.

Indeed, such is the power intrinsic in the Metta Sutta. Whoever with firm faith will recite the sutta, invoking the protection of the deities and meditating on metta, will not only safeguard himself in every way but will also protect all those around him, and will make spiritual progress that can be actually verified. No harm can ever befall a person who follows the path of metta.

Excerpted from Metta: The Philosophy and Practice of Universal Love, by Acharya Buddharakkhita © Buddhist Publication Society.)

The Karaniya Metta Sutta
The discourse is divided into two parts. The first detailing the standard of moral conduct required by one who wishes to attain purity and peace, and the second the method of the practice of metta.

He who is skilled in (working out his own) well-being, and who wishes to attain the state of calm (Nibbana) should act thus: he should be dexterous, upright, exceedingly upright, obedient, gentle, and humble,

Contented, easily supportable, with but few responsibilities, of simple livelihood, controlled in the senses, prudent, courteous, and not hanker after association with families.

Let him not perform the slightest wrong for which wise men may rebuke him. (Let him think:) “May all beings be happy and safe. May they have happy minds.”

Whatever living beings they may be — feeble or strong (or the seekers and the attained) long, stout, or of medium size, short, small, large, those seen or those unseen, those dwelling far or near, those who are born as well as those yet to be born — may all beings have happy minds.

Let him not deceive another nor despise anyone anywhere. In anger or ill-will let him not wish another ill.

Just as a mother would protect her only child with her life even so let one cultivate a boundless love towards all beings.

Let him radiate boundless love towards the entire world — above, below, and across — unhindered, without ill-will, with enmity.

Standing, walking, sitting or reclining, as long as he is awake, let him develop this mindfulness. This, they say, is “Noble Living” here.

Not falling into wrong views — being virtuous, endowed with insight, lust in the senses discarded — verily never again will he return to conceive in a womb.