Is Siddhartha Gautama the Buddha?
Yes he is THE Buddha. There can be many Buddhas but when we refer to THE Buddha, we usually refer to Siddhartha. Also, not everyone can become a Buddha. In fact very very very very very very very very very very very very very FEW have any chance. Wannabe Western Buddhists like to believe anyone can become a Buddha which is nonsense. To be a Buddha takes the kind of skill that is impossible to achieve by most. Yes there were Buddhas before Shakyamuni and will be after him as well. But they are extremely rare. The next Buddha will be Maitreya and be assured none of these western Buddhists stand a chance at being him. So THE Buddha or LORD BUDDHA only refer to Siddhartha. There are other Buddhas like Amitabha, Aksobhya, Amogasiddhi and Ratnasambhava. They are extremely special and the average Buddhist has NO CHANCE of ever joining their pantheon. THE Buddha or Siddhartha founded Buddhism and is known as the historical Buddha.
