In an extract from his new book, Rabbi Dr Naftali Brawer looks at the key High Holy Day theme of repentance.

The idea of a divine soul is central to the Jewish understanding of man, not only in the sense just described but also because it means that, at his core, man is pure and good. The body is temporary, as is the animal soul; the godly soul is eternal and, in that sense, the essence of who we really are.

This idea plays itself out in relation to sin and repentance. From a Jewish perspective no sin can ever blacken the soul. The soul may be denied expression by a particular sinful host but the soul itself, as an expression of God, remains untainted by sin.

This is the basis for the Jewish idea of teshuvah, which is erroneously translated as repentance when it is actually something else entirely. A Jew does not repent; he does teshuvah, which means he returns.

The idea of return can be understood in two ways. One is that man returns to God. Through his sins he becomes distant from his Father in Heaven and through regret and a resolve to change he returns to God. A second understanding of the meaning of teshuvah is that the sinner returns to himself; to his pure essence and the core of his being - in other words, to his soul. This is because, at his core, a person is good and pure.

There are many unsavoury and animalistic aspects to our personalities but those aspects are superficial in relation to our core soul, which is divine.

The Talmud introduces us to one of its most enigmatic characters, Elisha ben Avuya. Elisha started out as a budding rabbinical scholar who could count amongst his colleagues the famous Rabbi Akiva and amongst his disciples the famous Rabbi Meir.

While the Talmud is not entirely clear about how it happened, and there are various accounts, it is clear that at some point Elisha lost his faith - and lost it in spectacular fashion. It was not enough for him to slink away from organized Jewish life; he had to turn his heresy into a spectacle. He became a high-profile heretic and caused his former colleagues no end of heartache as a result.

At this point the Talmud stops referring to him by his real name and begins to call him Acher, which means "another one", as if to suggest that he is no longer the man once known and respected. Acher had a particularly complex relationship with his former disciple Rabbi Meir, who despite his master's obvious rejection of the faith still continued to refer to him as his rabbi.

It appears that for all Acher's bravado, or perhaps because of it, he was not content with his life and secretly harboured hope of finding some form of reconciliation with his abandoned faith; but somehow he could never bring himself to do so.

The Talmud relates how he appeared to Rabbi Meir on the holy Shabbat whilst riding a horse, a clear rabbinical violation. Rabbi Meir, who was respected in his town as a great rabbi, went out to greet his former master. The Talmud describes how Acher continued to ride his horse whilst Rabbi Meir walked alongside him on foot, urging his master to return to the faith.

As they approached the outskirts of town Acher remarked to Rabbi Meir that to go any further would be a violation of Shabbat (one cannot leave a city's limits on Shabbat) and that Rabbi Meir should return. "Rabbi," said Rabbi Meir, "you return with me."

Acher declined and went on his way. Rabbi Meir for his part never stopped hoping that one day his master would return and resume his old identity. Alas, this was not to be.

This tragic story is important because it demonstrates the power of teshuvah. Though, in fact, Acher never returns, Rabbi Meir believed that he could. His request of Acher to return with him is not a request to return to the town from which they departed but to Acher's own core self. This faith in the power of teshuvah even for as high a profile heretic as Acher is not misplaced. Acher ignored the call; but what is clear from the Talmud is that, had he chosen to heed it, he would have found his way back.

There is another story, much more contemporary, illustrating the Jewish view that a soul can never be tainted and that it retains its purity always. In the 1960s the Lubavitcher Rebbe, a great Chasidic master, sent one of his Chasidim to some far-flung place in the hope of discovering lost Jewish souls.

The Chasid assumed this mission with vigour and after some time returned to report his finding to the Rebbe. He had indeed located a few Jews and he was appalled at their lack of basic Jewish literacy. He had begun to instruct them and after a while found that he had succeeded in returning them to their faith. Searching for an apt metaphor he described his work as that of a ritual scribe who fixes faded letters on a Holy Torah scroll, the faded letters being the lost souls. The Rebbe looked up in astonishment: "Faded letters, you say? They are not faded letters. They are like the holy letters of the Ten Commandments etched in stone.

Letters engraved in stone never fade. At worst they get covered in dust. All that is required is for someone to come along and blow the dust away and there you will see the holy letters in all their majesty and glory."

Man is a strange hybrid, between something lower than an animal and more elevated than an angel. He is unlike either in that he possesses the divine gift of free choice whereas animals and even angels do not. Of all of creation he alone can choose his destiny. He is capable of the most unspeakable cruelty as well as the loftiest and most noble acts. Either way, it is he and he alone who bears responsibility for his actions.

It is not the easiest existence; indeed the rabbis said, "how much easier would it have been had we never been born.' Yet it is a most challenging and rewarding existence. It is also the most uplifting as we alone live with the knowledge that deep inside us dwells a spark of God.

From "A Brief Guide to Judaism - Theology, History and Practice", Naftali Brawer, Robinson, £8.99, to be published on October 16