The Fourth Noble Truth
"There are these two extremes that are not to be indulged in by one who has gone forth. Which two? That which is devoted to sensual pleasure in connection with sensuality: base, domestic, common, ignoble, unprofitable; and that which is devoted to self-affliction: painful, ignoble, unprofitable. Avoiding both of these extremes, the middle way realized by the Tathagata — producing vision, producing knowledge — leads to calm, to direct knowledge, to self-awakening, to Unbinding.
"And what is the middle way realized by the Tathagata that — producing vision, producing knowledge — leads to calm, to direct knowledge, to self-awakening, to Unbinding? Precisely this Noble Eightfold Path: right view, right resolve, right speech, right action, right livelihood, right effort, right mindfulness, right concentration. This is the middle way realized by the Tathagata that — producing vision, producing knowledge — leads to calm, to direct knowledge, to self-awakening, to Unbinding."
The Fourth Noble Truth states that one must follow the Eightfold Path in order to reach awakening. Now, as you will see - if one lacks belief in rebirth, you cannot even fulfil the very fundamentals of Buddhism. Why is this?
Because belief in Kamma is essential for one to posses Right View. 'Mundane right view involves a correct grasp of the law of kamma' [The Noble Eightfold Path - Bhikku Bodhi page 15] can one grasp the law of kamma correctly without believing in rebirth?
The answer is no. Kamma needs rebirth to make sense. Kamma being that our actions bear fruit at some time or another. One lifetime simply doesn't allow this to happen. Forgive for bringing Hitler into the argument, but I hope I shall be forgiven, because I am not likening anyone to him, just merely pointing out an obvious example. Hitler was responsible for the murder of 6 million Jews, and countless more deaths in battle, do you think that Hitler's actions bore fruit in this life? He was not brought to justice, nor even slain by an enemy bullet, but his own bullet, with his own finger on the trigger. Did he really have to bear any kind of proportional negative kammic fruit for his actions? I do not believe that he did. So kamma would have failed. Not all volitional actions reap fruit if confined to one life time.
The same argument can be used in reverse. If we are not subject to rebirth, then we are not born with any kamma, so what kind of negative kamma must all those Jews have accrued within one life time to be able to say that their suffering and deaths at the hands of the Nazis were the result of kammic fruit? What could all those people possibly have done? Kamma does not work as an explanation here, without rebirth [not that it is really an explanation with rebirth, but that's irrelevant for the moment].
To be a Buddhist and follow the Eightfold Path correctly, you must at least entertain the notion of rebirth as a possibility, for if you deny it then you do not have a correct grasp of kamma, and you would be holding of a wrong view (according to the Buddha).
"For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring."