Rabbi Leo Baeck presented his major philosophical ideas in a book called The Essence of Judaism. It was titled as a response to Adolph von Harnack's book The Essence of Christianity, which Baeck critiqued when it was published in 1901.
Leo Baeck accepted Hermann Cohen's idea of ethical monotheism as the essence of Judaism but added mystery, a sense of the holy, as a second Jewish essence. Unlike Cohen, who found the idea of God sufficient, Leo Baeck wrote about our emotional awareness, our experiencing the Divine. This awareness naturally leads to ethical acts, what Baeck called the commandments. At the same time, we can also maintain peoplehood, a sense of our role in history, through ritual acts, which give expression to our sense of mystery.
Baeck, a liberal modern Jew, was not prepared to assign authority to the ritual acts, only to the ethical imperatives. Unlike Cohen, however, he was not willing to give up the reality of God for Cohen's "idea." God is both transcendent and immanent. For Baeck, God is real. Therefore, being Jewish consisted of being ethical and striving for universal good, experiencing the mystery of the Divine, and maintaining the survival of Jews throughout history.
By emphasizing experience and mystery as part of the essence of Judaism, Baeck moved radically away from Cohen's "religion of reason." However, Baeck's emphasis on universal ethics and ethical monotheism made him a bridge between the rationalists and the modern Jewish existentialists.
Baeck's personal life deserves some mention because he lived by the values described in his writings. As president of the representative body of Jews in Germany after 1933, he was given many opportunities to escape. He refused, insisting that he would stay so long as there was a minyan in Germany. In 1943 he was sent to the Terezin (Terezienstadt) concentration camp. He survived the horrors by helping others, teaching, and refusing to lose his sense of self or dignity. His philosophical beliefs were not swayed by the Holocaust. He always maintained that evil was the result of humans using their free will to not do the ethical. The enormity of the Nazi atrocities did not shake that belief.
Sources: Gates to Jewish Heritage