He became Moral Tutor at Jews' College, London, where he taught Talmud and homiletics during the last years of Rabbi Isidore Epstein's tenure as principal. By this time Jacobs had drifted away from the very rigid traditional approach to Jewish theology that had marked his formative years. Instead he struggled to find a synthesis that would accommodate Orthodox Jewish theology and modern day higher biblical criticism. Jacobs was especially concerned with how to reconcile modern day Orthodox Jewish faith with the documentary hypothesis. His ideas about the subject were published in a book entitled We Have Reason to Believe, published in 1957. The book was originally written to record the essence of discussions held on its title's subject at weekly classes given by Jacobs at the New West End Synagogue and was the subject at the time of some mild criticism, but not of any major censure.
A true Jewish apologetic, eschewing obscurantism, religious schizophrenia, and intellectual dishonesty, will be based on the conviction that all truth, ‘the seals of the Holy One, blessed is He’, is one, and that a synthesis is possible between the permanent values and truth of tradition and the best thought of the day.
Jacobs therefore places himself in the line of expositors from Moses Mendelssohn onwards who have sought to reconcile, or at least clearly contextualise, the concepts of Judaism with the prevalent thought and society of the modern world. The book is illustrated throughout with references and quotations from authorities both ancient and modern, both Jewish and Gentile, reflecting Jacobs’s broad interests and reading.
Most of the book, dealing with such topics as proof of God’s existence, pain, miracles, the after-life, and the idea of a ‘Chosen People’, is full of stimulating ideas which were however not in themselves controversial. Debate on the book was eventually to centre on chapters 6, 7, and 8: The Torah and Modern Criticism, A Synthesis of the Traditional and Critical Views and Bible Difficulties.
In these chapters Jacobs took on discussion of ‘Modern Criticism’ of the Bible, more specifically textual analysis of the Torah known as the ‘Documentary Hypothesis’, which suggests that its texts derives from multiple sources, rather than having been given, as Orthodox Rabbinical traditions have it, complete in its present form by God to Moses during the period beginning on Mount Sinai and ending with Moses's death.
Jacobs comments: 'While Judaism stands or falls on the belief in revelation, there is no ‘official’ interpretation on the way in which God spoke to man' He points out that ‘according to some Rabbis, [the Pentateuch] was given to Moses at intervals during the sojourn in the Wilderness’. But he also points out that given the arguments of textual criticism ‘no work of Jewish apologetics, however limited in scope, can afford to fight shy of the problem’. Here there is an implied rebuke of the tendency of many Jewish authorities of the period simply to gloss over the inconveniences of the thoughts of the ‘modern critics’ – a rebuke which perhaps rankled with some.
Jacobs continues with some considerations of textual criticism by treating of the process of Masoretic emendation, which has proved acceptable and even desirable. This acceptability itself negates the idea, most ably expounded by Maimonides, of the perfection and immutability of the written text of the Torah; from which Jacobs concludes ‘there is nothing to deter the faithful Jew from accepting the principle of textual criticism’. He is aware that ‘to talk about ‘reconciling’ the Maimonidean idea and the Documentary Hypothesis […]is futile, for you cannot reconcile two contradictory theories. But to say this is not to preclude the possibility of a synthesis between the old knowledge and the new knowledge’. .
Jacobs provides numerous examples from the Talmud and from other rabbinical writings indicating acceptance of the idea of Divine intervention in human affairs, with ‘God revealing his Will not alone to men but through men’. He concludes that, even if the Documentary Hypothesis is partly (or even entirely) correct, ‘God’s power is not lessened because He preferred to co-operate with His creatures in producing the Book of Books […] We hear the authentic voice of God speaking to us through the pages of the Bible […] and its message is in no way affected in that we can only hear that voice through the medium of human beings’.
The British newspaper, The Jewish Chronicle, took up the issue and turned it into a cause celebre which was reported in the national press, including The Times. When Jacobs wished to return to his pulpit at the New West End Synagogue Brodie vetoed his appointment. A number of members then left the New West End Synagogue to found the New London Synagogue.
While holding the position of Rabbi at the New London Synagogue, Dr. Jacobs was also for many years Lecturer in Talmud and Zohar at the Leo Baeck College, a rabbinical college preparing students to serve as Conservative, Reform and Liberal rabbis in the UK and Europe. Rabbi Jacobs served as Chairman of the Academic Committee for some years.
Since the founding of the New London Synagogue, Jacobs and the Masorti movement were subject to consistent hostility from Orthodox British Jewish institutions. On his 83rd birthday, in the Bournemouth (Orthodox) synagogue on the sabbath before his granddaughter's wedding, Jacobs was not provided the honour of an aliyah customarily given to the father of the bride, which gave rise to heated correspondence in the Jewish press including accusations of pettiness and vindictiveness. The Chief Rabbi, Sir Jonathan Sacks, and the head of the London Beth Din, Dayan Chanoch Ehrentreu, responded that, because of what they considered to be Jacobs's heretical beliefs, "they believed that had Jacobs uttered the words 'Our God […] who gave us the Torah of truth […] ', he would have made a false statement.
In December 2005, a poll by the Jewish Chronicle of its subscribers, in which 2000 readers made their nominations, voted Jacobs the 'greatest British Jew' in the community's 350-year history in England. Jacobs commented 'I feel greatly honoured - and rather daft.' A criticism levelled at the poll, was that the Jewish Chronicle has always championed Louis Jacobs, and non orthodox causes, and subscribers would have subscribed to those views. Nevertheless, the story that Louis Jacobs had been nominated greatest British Jew, received wide press coverage in Britain.
A few months before his death he donated his great book collection to the Leopold Muller Memorial Library at the Oxford Centre for Hebrew and Jewish Studies.