In the third watch of Shakyamuni's night of Enlightenment, the Four Noble Truths were revealed to him, the fourth being that there is a definable path, or method, by which Enlightenment is obtained. If one were to persist in doing 8 things in a correct, and not in a wrong manner, then the result would be Nirvana.
The 8 items also had an order to them, and the 8th was samadhi or concentration, that is focusing the mind on a single point. The first 7 were the necessary supports for the 8th, as he explained in a discourse later in his life. In his own quest, in the 6 years from the time he left home until he attained enlightenment, his single point of focus was the concept of Nirvana. While leading a family life and pondering over the lot of humanity, it was the witnessing the serenity of a monk questing for Nirvana which triggered his decision to leave home at age 29.

Initially he knew Nirvana by definition as the complete extinction of the senses, implying a resulting state of absolute inner happiness, yet neither he nor anyone else knew Nirvana by experience. He wandered and begged, he meditated in solitude; he joined the communities of several venerable teachers, and he mastered a whole range of ascetic practices, one by one. But upon reaching mastery of a particular meditation or teaching or ascetic practice, he questioned whether or not this was Nirvana or would lead to Nirvana, and as the answer was negative, he quickly abandoned the practice and moved on to another.
Although for 6 years Shakyamuni single-pointedly sought the state of Nirvana and refused to rest in anything else, he later incorporated many elements learned during this time into his body of teachings, such elements as:

„h leaving home
„h wandering
„h keeping rules of discipline
„h practicing the 4 absorptions
„h entering the meditative state of emptiness
„h infinite space
„h infinite consciousness
„h neither perception nor non-perception
„h living in community
„h and undertaking (mostly at the option of the monk)12 possible ascetic practices.

But it was the concept of Nirvana that served as Shakyamuni's point of concentration until it was finally realized. Then after dwelling in Nirvana for a couple of days, a profound change took place in which he decided to temporarily relinquish this state in order to benefit others with his knowledge and wisdom. From that time on, for the next 40 years, he taught people the 4 Noble Truths and the 8 Fold Noble Path in their innumerable facets, often creatively meeting people on their own ground as the starting point, then leading them to realize their errors, inspiring them and revealing the genius of the Buddha. The Buddha, knowing the state of Nirvana, can return there at will, and does so frequently during deep meditation, but at other times interacts with the world, retaining mindfulness and proscribed conduct so as to maximize the benefit to the people, and not get caught, or contribute to the world's net of sufferings.

By the time of the Buddha's passing away, approximately 200 people, mostly monks, but also nuns, lay men and lay women were recorded as having reached Nirvana. Among them were 16 great disciples pointed to as exemplifying 16 different areas of expertise, showing that right from the beginning there was wide diversity in the Sangha. In the subsequent generations among the diverse lineages, there were those who, in their own deep meditations, understood that the path of Buddhahood was a step beyond Nirvana. Those who sought to know Buddhahood and emulate the Buddha's path were called Bodhisattva's, as opposed to those meditators who only sought Nirvana for themselves.
It is the Buddhist path of the Bodhisattva which carries the Dharma down to us 2500 years later, emphasizing entry into samadhi, starting with the samadhis of Samatha, bringing tranquility, then moving into Vipasyana, translated as "discerning the real", and concentrating on the Buddha. This concentration on the Buddha (Buddha - anusmirtu)is the common thread at the heart of the great Mahayana Sutras: the Lotus Sutra, the Avatamsaka Sutra, and the Pure Land Sutras.

Hisao Inagaki, in the lengthy commentary to his translation of the Pure Land Sutras (1995) writes,

" This samadhi was shared by disciples of superior spirituality who in turn transmitted it to later generations. Thus for a few centuries [after Shakyamuni] the essentials of the Pure Land teachings were transmitted through samadhi, until it was codified and translated into the languages of different periods and places. . .. Although there is no way of knowing how those samadhi's centering on [the Buddha, in this case] Amitabha were transmitted in India, there is little doubt that this is more fundamental and older than the transmissions of written scriptures. It should also be noted that such transmission is the sure and quick way of conveying the teaching."

The scriptures then refer back to this samadhi by emphasizing the practice of Nembutsu (remembering the Buddha)through mantra recitation. The great masters of this tradition made it their habit to recite the mantra many thousands of times a day. For example, Honen practiced 60,000 mantra recitations daily.

In the Lotus Sutra, the original core, according to scholars, was chapters 2 - 9, with other chapters added at different times later. And here, in chapter 2, the primary of entering into contemplation of the Buddha is strongly stated:
"Sariputra! What is the one great purpose for which the Buddhas . . . appear in the worlds? The Buddhas . . . appear in the worlds in order to cause all living beings to open the gate to the insight of the Buddha, and to cause them to purify themselves. They appear in the worlds in order to show the insight of the Buddha to all living beings . . . to cause all living beings to obtain the insight of the Buddha . . . to enter the Way to insight of thew Buddha . . ."

And in the Gandaryuha, the final book of the Avatamsaka Sutra, there are maybe several thousand samadhis specifically mentioned in the text, but in the fundamental one, Shakyamuni "wanting to establish those Bodhisattvas in this lion-emergence concentration of Buddhas," and to show them "all ways into the reality realm," reveals the Buddha Vairocana, and there upon, the Bodhisattvas "entered the oceans of mystical projections of Buddhahood of the Blessed Vairocana."
Furthermore, when the central figure in the sutra, the pilgrim Sudhana, visits the first of 53 spiritual benefactors, Meghashri tells him, "I have attained mindfulness of the Buddhas, . . . I see the Buddhas in all the lands of the 10 directions." From this fountainhead of concentration on the Buddha, all the myriad of other samadhis flow, all the various expedient means of benefiting sentient beings.
For the person in whom, in the midst of modern civilization, the aspiration for enlightenment has arisen, that is the aspiration to renunciation, the aspiration to love, and the aspiration to harmlessness, there is a procedure to be followed in order to learn the samadhi on the Buddha. Even as Sudhana did, one will place himself in charge of spiritual benefactors. After giving refuge in the Triple Treasure and giving precepts, they will put you through some purification exercises, set you on a course of learning the Dharma, teach meditation and other practices, and establish you in the community of meditative life.

Beyond this is the actual method involving the three simultaneous factors of body, speech and mind. The type of speech known as mantra and dharani is the key to attaining the necessary samadhi. The "mind-to-mind transmission" of Zen is also samadhi on the Buddha. However, by relying on body and mind, but not on speech, the realization is made much more difficult. Mantra is so vital in itself that a whole separate branch of Mahayana developed around it, calling itself Vajrayana, Mantrayana, Mikkyo, etc. And concurrently, as is natural, a whole set of problems unique to the Mantrayana arose. The major one, probably, is that by conceiving of itself as a "higher path," it sometimes belittled the non-Mantrayana teachings, thereby opening up the possibility of ignoring its own fountainhead, that is the Buddhahood of Shakyamuni, his instructions recorded in the Agama Sutras, and even the Bodhicitta path, which was born from practicing the dharma according to Shakyamuni.
Then being aware of the pitfalls of the Vajrayana, one relies on one's lineage of teachers, works diligently with mantra, and at some point, becomes able to enter the vast and beautiful world of the Buddhas, from where one sees the samsaric world with love and compassion for all creatures, great and small, arguing with no one, merely stating the truth intent upon the infinite gift of Shakyamuni.