Universalism as the term is used in religion refers to two concepts; either (1) the idea that a religion is meant for all humanity rather than just a certain race, tribe, caste or gender, or (2) the belief that everyone, no matter what their religion, will be saved, enlightened or liberated. The opposite of universalism is particularism.
In this first sense, Buddhism is the oldest universalist religion. The Buddha described himself as `a teacher of gods and humans' (satthà devamanussànaü) i.e. of all beings capable of reasoning and comprehending. Once he said that even the trees would embrace the Dhamma if they had the ability to comprehend, `how much more so human beings?' (A.II,194). After he made his first disciples, he instructed them to proclaim the Dhamma for `the good of the many for the happiness of the many, out of compassion for the world' (Vin.I,20). The Buddha's universalism is particularly striking considering that the Brahmanism of the time was so strongly particularist. Hindu scriptures and law books insist that low caste and outcaste people and foreigners (mleccha) are forbidden to read the scriptures, participate in sacred rites or even enter temples, although many modern Hindu reformers now repudiate such restrictions.
In the second sense that the term universalism is used, Buddhism was also the first and still one of the few universalist religions. The Buddha once said: `There is no true ascetic outside' (samaõo natthi bàhire, Dhp.254). This has sometimes been interpreted to mean that outside (bàhira) Buddhism no one can attain enlightenment. However, all it actually says is that, other than the Buddha's ordained disciples, no other monks or nuns qualified to be genuine ascetics, which may well have been the case at the time the Buddha said it. Someone once asked the Buddha if any of the monks and priests of other sects or religions had attained enlightenment (Sn.1081). He replied: `I do not say that all monks and priests are shrouded in birth and death (i.e. saüsàra). Whoever does not cling to sense experience or morality and rules, has given up doubts, is free from craving and defilements, I say that one has attained nirvana' (Sn.1082). Thus the Buddha's answer was not a sweeping assertion that only within his Dhamma can someone attain final liberation, but rather an `it depends'. On another occasion someone asked the Buddha if he denied that those of other religions could become arahats, i.e. attain enlightenment. He replied: `I do not deny that others can become arahats.' (Na khoarahattassa maccharàyàmi, D.III,7). In another discourse the Buddha asserted that some individuals `attain the unalterable path' (okkamati niyāmaṃ) that leads to enlightenment even if they never see him or hear his Dhamma (A.I,121).
The attainment of enlightenment is not dependent on winning the approval of a deity, but on realizing certain natural truths, which everyone has the capacity to do. This being the case, it is conceivable that even those who have never even heard the Dhamma could become enlightened. However, this could be said. Openness to the Buddha's teaching makes an appreciation of it more likely. Appreciation of the Buddha's teaching would make the desire to practise it more possible. Practicing the Buddha's teaching would make attaining enlightenment many times more probable.