Out of great compassion, Lord Buddha set forth three wheels of law to suit varying degrees of intelligence and receptivities. He revealed many different means to attain enlightenment and win liberation from the cycle of birth and death. At the Dear Park near Varanasi, he formulated his doctrines in the form of Four Noble Truths and designated as Sravakayana.
At the Vulture's Peak near Rajgir, he expounded the doctrine of emptiness and other Vaipulya Sutras and this approach constituted as Bodhisattva tradition or Mahayana.
At Vaisali he preached the doctrine of Buddha Nature in Sandhinirmocana Sutra.At various places such as Dhanyakataka, Sriparvata,Kamakhya, Sirihatta etc, including higher planes of existence, Lord Buddha revealed the methods of Mantrayana to his highly gifted disciples as shorter path to attain Enlightenment. This approach was termed as Vajrayana which integrates all three vehicles.
The casual reader or one who has not studied or fully realised the diverse nature of doctrines of Lord Buddha may find himself quite puzzled by seemingly contradictory statements between Sravakayana and Mahayana and also between Sutraand Tantra. But when one acquires a comprehensive knowledge of the methods and stages of the paths of the enlightenment, one finds that there are no contradictions in the teachings of Lord Buddha. Through the study of the Bodhipathapradipa of Atisha:
a) You will understand that in all the Buddha's teachings there are no contradictions.
b) You will be made aware that all the scriptural texts are to be taken as sound advice, as there is no contradiction between the text and the practice.
c) You will then discover the significance of the 'Three Themes' of the Buddha's teachings, i.e., renunciation, Bodhicitta and the correct view of Sunyata.
d) You will be protected from the abyss of the 'Great Mistake' (sectarian views).
The study of the stages of the path to enlightenment has these four benefits.
Lord Buddha gave various teachings to different individuals according to their capacities. Just as a medical doctor prescribes various diets in accordance to the progress of patient's health, so does a person practices these various methods as his mind develops progressively during his spiritual journey towards enlightenment. So within the Buddhist system, although Anuttara Yoga Tantra is the highest system, the Buddha did not teach Anuttara Yoga to every disciple. He would indeed have done so if every one were capable of practising it. To those for whom Annuttara Yoga Tantra was not suitable, he taught Yoga Tantra. To those for whom even this was not suitable, he taught Carya Tantra. To those for whom this was not suitable, he taught Kriya Tantra. To those for whom this was not suitable, he taught the Sutras. Even within the Sutra tradition, there are four views on the cognition of emptiness. He taught the Prajnaparamita teachings to set forth Madhyamika views. To those for whom this was not suitable, he taught Sutras like Lankavatara, Sandhinirmocana to express his Cittamatra (mind only) view. Again he set forth the vehicle of Pratyeka Buddha and even more, the vehicle of Sravaka.
Even within Sravaka tradition, there are seven sets of vows for monks and lay persons. The vows of the lowest type of lay persons consist in the acceptance of five precepts or simply the maintenance of the vows of the triple refuge. Sakyamuni Buddha instructed his disciples to consider Samsara as a disease, the Dharmas a medicine, the teacher as a doctor and the results of the practice as the cure.
As there are different diseases, we cannot tell that all patients must take one particular medicine. In taking different foods by different people, we do not have any dispute because one takes what is suitable for him. Similarly, we should not disparage the various teachings of the Buddha. This would mean disparaging the Buddha himself. Thus it would be wise not to have a sectarian views.
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