Do you have to believe in rebirth, other worlds and post-mortem kamma to practise Buddhism?

To start practising, no: the Theravāda suttas require no special oath and no unconditional faith. The moral precepts (sīla) coincide with ordinary human morality, and the practices of concentration and mindfulness are meant for this present life. But the ideas of rebirth and kamma are firmly built into the early teaching. They cannot be removed without damaging the rest of the structure. It would be harder still to do so for Mahayana and Vajrayana Buddhism.

Faith and kamma

Kamma in the suttas is first of all intentional action (cetanā): some of its results are visible already in this life, and some only after it. So the ideas of kamma, of rebirth and of faith in both are closely intertwined. But “faith” in Buddhism is noticeably different from what Western readers are used to. In MN 95 the young brahmin Kāpaṭhika asks the Buddha about the Vedic tradition, which the brahmins consider the only truth because of its antiquity. In reply the Buddha introduces the idea of “preserving the truth” (saccānurakkhaṇā): if a practitioner has faith (saddhā) in some conviction, adding the phrase “this is my faith” makes the speaker’s words true. But saying of a conviction “only this is true, everything else is false” can easily be a mistake. So Buddhist faith, saddhā, is rather informed and conscious trust. This is exactly how Buddhism invites you to believe in kamma and rebirth - simply because it pays off. In AN 3.65, for example, a noble disciple (ariyasāvaka) whose mind is cleansed of hostility and ill will gains four assurances:

• his position remains a winning one if there is another world with the fruit of deeds (kamma);
• his position remains a winning one even if there is no other world;
• if evil befalls the one who does evil, then suffering does not threaten the one who does no evil;
• and if evil does not befall the one who does it, the one who does no evil remains pure all the same.

Rebirth

The idea of rebirth is built into the canon as firmly as kamma and other worlds. MN 117 distinguishes two kinds of right view:

• The mundane kind (sāsavaṃ puññabhāgiyaṃ upadhivepakkaṃ - “affected by taints, partaking of merit, ripening in the acquisitions”): there is what is given and what is offered, there is fruit of good and bad actions, there is this world and the other world, there is mother and father, there are beings reborn spontaneously, there are ascetics who have realised both worlds themselves.
• The noble and supramundane kind - a factor of the path, the wisdom that discriminates.

Despite all of the above, we do not know what awaits us after death. So it makes sense to work (and to work now!) with what can be checked - greed, hatred, attention, actions and their consequences. At the same time, it is worth remembering that the four truths in the Pali Canon are not confined to one life. Suffering there is defined through birth, ageing and death, and the goal is the ending of further becoming. So early Buddhism is by no means neutral about rebirth.

Where to read: MN 60, AN 3.65, MN 95, MN 117.