Someone posed the question "What is a 'Buddhist'?" on the Merely Transitional Facebook Page. This particular question once was also asked in an exam I had to do and had to write an answer of at least 1 page. Back then I had to provide a clear cut scientific answer, but this time I will give a little shorter answer.
Strictly speaking, someone is a Buddhist if he/she calls him/herself so. Since the Dhamma is open and accessible regardless peoples' personal background, there are no strict rules set up for someone who wants to become a Buddhist. One does not require a certain declaration, speak out some kind of formula, in order to become a follower of this particular religion - unlike, for example, in Islam where one needs to make the statement of shahadah in order to acknowledge God. Or like in Christianity where one ought to be bapitized before one actually is granted the relationship with God (through Jesus).
But then again, if we were to distinguish the Buddhist laity from its monastic order, then, indeed there are similarities to various other religious (monastic) orders. Because before one can become a monk, he/she first needs to get ordained as a novice. Without going into too much detail here about the several preliminary phases of Buddhist monastic life, suffice it to say that at the beginning of each phase one receives also a Buddhist name (in Pali). This new name that one is given also serves as a statement of one's faith while at the same time it may be used as a sign of one's (socio-religious) authority, too.
So, what then is a 'Buddhist'; in a way, it encompasses the whole community, not just the monastic order but the laity as well, for they are inseparable from the community as a whole, and moreover, their support is indispensable for the monks. Yet, in another way, being a Buddhist is reserved for a certain elite; the members of the monastic order, whom all have undergone and passed through a set of preliminary rituals and practices before they "earned" their Buddhist "title", which, thereby may come to function similarly to scholarly degrees obtained in University.
It is important, however, to remember that at the time of the Buddha there was no Buddhism yet, let alone a Buddhist monastic community. The Buddha himself, when he decided to renounce his worldly life he took off his royal garment and put on pieces of discarded cloth. This particular act is, of course, symbolical in the sense that he no longer cherished all that he previously had enjoyed during his lifetime. Thus, he became a wandering mendicant, and not a monk, for there was no such order established yet. He also was not told by such an order to undergo any preliminary stages on the path before actually becoming accepted as member or follower of the religion. In fact, all that was needed to throd on the path of the holy life (brahmacariya) was a symbolical renunciation of one's worldly possessions, and a simple and short declaration of the sentence "Ehi bhikkhu, svakkhato dhammo caro brahmacariyam samma dukkhassa antakiriyaya" (Come bhikkhu, well-expounded is the Dhamma, live the holy life for the complete ending of suffering). So, basically anyone could become a mendicant by reciting this particular sentence. But most importantly, after one had recited this phrase, one was not considered a 'Buddhist', but rather as someone living the holy life (brahmacariya).
Later the number of these mendicants came to grow significantly, which, over time, lead to more complex procedures of ordination. Once the rules had become standardized in accordance to the Vinaya of the established monastic order, the newly ordained formally became a Buddhist (monk). This still is the common rule of practice in Buddhism today. Considering this, we should perhaps rephrase the question of "what is a 'Buddhist'" by looking at what it is we try to define here. The word 'Buddhist' is merely a concept conditioned by socio-religious perception. But if we were to attempt what actually is meant by being a 'Buddhist', then soon enough we will find that mere concepts will prove inadequate to provide a satisfactory interpretation of the true meaning implied. The truth is, wars are being fought because of peoples' various interpretations and understanding of religious concepts, which all too often leads to doctrinal fanatism and dogmatic fundamentalism. So, the important thing is not create division through emphasis of certain concepts, but to recognize the abundant similarities among the seemingly differences in religion and its various teachings.
One becomes Noble by leading the holy life, not by mere following a certain religion, or let's say by becoming a Buddhist. Noble persons are not restricted to the Buddhist religion alone, likewise, not all Noble persons are necessarily Buddhist. A Buddhist, then, is just the same like any follower of any particular religion; they all strive to live the holy life - whatever that may mean to him/her depending on the given circumstances, such as the person's socio-cultural background - which, in both Buddhism and Hinduism happen to be refered to as 'brahmacariya'.
