Rabbi Abraham Joshua Heschel's yarzheit was last week. Every modern Jew should know him.
Heschel explicated many facets of Jewish thought including studies on medieval Jewish philosophy, Kabbalah, and Hasidism. According to some scholars, he was more interested in spirituality than critical text study, which was a specialty of many scholars at JTS. He was not given a graduate assistant for many years and was relegated to teach mainly in the education school or Rabbinical school, not the academic graduate program. Heschel was particularly looked down upon by his colleague Mordechai Kaplan, founder of Reconstructionist Judaism, and many students who attended JTS in the 50s sympathized with Kaplan over Heschel. 
Heschel saw the teachings of the Hebrew prophets as a clarion call for social action in the United States and worked for black civil rights and against the Vietnam War Heschel was an activist for civil rights in the United States.
He also specifically criticized what he called "pan-halakhism", or a focus upon religiously-compatible behavior.
Influence outside of Judaism
Heschel is among the few widely read Jewish theologians. His most influential works include Man is Not Alone, God in Search of Man, The Sabbath, and The Prophets. At the Vatican Council II, as representative of American Jews, Heschel persuaded the Roman Catholic Church to eliminate or modify passages in its liturgy that demeaned the Jews, or expected their conversion to Christianity. His theological works argued that religious experience is a fundamentally human impulse, not just a Jewish one, and that no religious community could claim a monopoly on religious truth.
This work started out as his Ph.D. thesis in German, which he later expanded and translated into English. Originally published in a two-volume edition, this work studies the books of the Hebrew prophets. It covers their life and the historical context that their missions were set in, summarizes their work, and discusses their psychological state. In it Heschel forwards what would become a central idea in his theology: that the prophetic (and, ultimately, Jewish) view of God is best understood not as anthropomorphic (that God takes human form) but rather as anthropopathic - that God has human feelings.
The Sabbath: Its Meaning For Modern Man is a work on the nature and celebration of Shabbat, the Jewish Sabbath. This work is rooted in the thesis that Judaism is a religion of time, not space, and that the Sabbath symbolizes the sanctification of time.
Man is Not Alone
Man Is Not Alone: A Philosophy of Religion offers Heschel's views on how man can apprehend God. Judaism views God as being radically different from man, so Heschel explores the ways that Judaism teaches that a person may have an encounter with the ineffable. A recurring theme in this work is the radical amazement that man experiences when experiencing the presence of the Divine. Heschel then goes to explore the problems of doubts and faith; what Judaism means by teaching that God is one; the essence of man and the problem of man's needs; the definition of religion in general and of Judaism in particular; and man's yearning for spirituality. He offers his views as to Judaism being a pattern for life.
God in Search of Man
God in Search of Man: A Philosophy of Judaism is a companion volume to Man is Not Alone. In this book Heschel discusses the nature of religious thought, how thought becomes faith, and how faith creates responses in the believer. He discusses ways that man can seek God's presence, and the radical amazement that man receives in return. He offers a criticism of nature worship; a study of man's metaphysical loneliness, and his view that we can consider God to be in search of man. The first section concludes with a study of Jews as a chosen people. Section two deals with the idea of revelation, and what it means for one to be a prophet. This section gives us his idea of revelation as a process, as opposed to an event. This relates to Israel's commitment to God. Section three discusses his views of how a Jew should understand the nature of Judaism as a religion. He discusses and rejects the idea that mere faith (without law) alone is enough, but then cautions against rabbis he sees as adding too many restrictions to Jewish law. He discusses the need to correlate ritual observance with spirituality and love, the importance of Kavanah (intention) when performing mitzvot. He engages in a discussion of religious behaviorism - when people strive for external compliance with the law, yet disregard the importance of inner devotion.
Prophetic Inspiration After the Prophets
Heschel wrote a series of articles, originally in Hebrew, on the existence of prophecy in Judaism after the destruction of the Holy Temple in Jerusalem in 70 CE. These essays were translated into English and published as Prophetic Inspiration After the Prophets: Maimonides and Others by the American Judaica publisher Ktav.
The publisher of this book states, "The standard Jewish view is that prophecy ended with the ancient prophets, somewhere early in the Second Temple era. Heschel demonstrated that this view is not altogether accurate. Belief in the possibility of continued prophetic inspiration, and in its actual occurrence appear throughout much of the medieval period, and even in modern times. Heschel's work on prophetic inspiration in the Middle Ages originally appeared in two Hebrew long articles. In them he concentrated on the idea that prophetic inspiration was possible even in post-Talmudic times, and, indeed, had taken place at various times and in various schools, from the Geonim to Maimonides and beyond."
Torah min HaShamayim
Many consider Heschel's Torah min HaShamayim BeAspaklariya shel HaDorot, (Torah from Heaven in the light of the generations) to be his masterwork. The three volumes of this work are a study of classical rabbinic theology and aggadah, as opposed to halakha (Jewish law.) It explores the views of the rabbis in the Mishnah, Talmud and Midrash about the nature of Torah, the revelation of God to mankind, prophecy, and the ways that Jews have used scriptural exegesis to expand and understand these core Jewish texts. In this work Heschel views the second century sages Rabbis Akiva ben Yosef and Ishmael ben Elisha as paradigms for the two dominant world-views in Jewish theology
Two Hebrew volumes were published during his lifetime by Soncino Press, and the third Hebrew volume was published posthumously by JTS Press in the 1990s. An English translation of all three volumes, with notes, essays and appendices, was translated and edited by Rabbi Gordon Tucker, entitled Heavenly Torah: As Refracted Through the Generations. In its own right it can be the subject of intense study and analysis, and provides insight into the relationship between God and Man beyond the world of Judaism and for all Monotheism.
* "Racism is man's gravest threat to man - the maximum hatred for a minimum reason."
* "All it takes is one person... and another... and another... and another... to start a movement"
* "Wonder rather than doubt is the root of all knowledge."
* "A religious man is a person who holds God and man in one thought at one time, at all times, who suffers harm done to others, whose greatest passion is compassion, whose greatest strength is love and defiance of despair."
* " God is either of no importance, or of supreme importance."
* "Just to be is a blessing. Just to live is holy."
* "Self-respect is the fruit of discipline, the sense of dignity grows with the ability to say no to oneself."
* "Life without commitment is not worth living."
* "Above all, the prophets remind us of the moral state of a people: Few are guilty, but all are responsible."
* "Remember that there is a meaning beyond absurdity. Be sure that every little deed counts, that every word has power. Never forget that you can still do your share to redeem the world in spite of all absurdities and frustrations and disappointments."
* "When I was young, I admired clever people. Now that I am old, I admire kind people."
* "Awareness of symbolic meaning is awareness of a specific idea; kavanah is awareness of an ineffable situation.
* "A Jew is asked to take a leap of action rather than a leap of thought."
* "Speech has power. Words do not fade. What starts out as a sound, ends in a deed."
* "The Almighty has not created the universe that we may have opportunities to satisfy our greed, envy and ambition."
* "The higher goal of spiritual living is not to amass a wealth of information, but to face sacred moments."
* "The course of life is unpredictable... no one can write his autobiography in advance."