Abraham Ecumene: Why Judaism, Christianity and Islam Worship the Same God.
Do Judaism, Christianity and Islam worship the same God? The answer is yes because of "Abraham Ecumene". All three of these Abrahamic religions positively affirm Abraham as their father and esteemed prophet: Christians regard Abraham as their spiritual father, and both Judaism and Islam view Abraham as their physical father. Abraham Ecumene means that these three religions are children in the same household of Abraham and worship the same God of their father Abraham. Abraham is not a common reference point between these three religions, because each religion makes exclusive claims about the life and faith of Abraham that cannot be harmonized between them, and may not be dismissed as insignificant. However, these conflicting claims may be explained as sibling rivalry between the children within Abraham's household, and do not undermine the truth that they are united in their admiration and affirmation of Abraham. Abraham continues to stand as a real starting place in a trialogue between the Abrahamic religions that demonstrates that they are unified in their worship of the same God of Abraham, and that Abraham continues to unify them, even when they are at war with each other.
Many Christians, Jews and Muslims object to the truth that they worship the same God as the other Abrahamic religions, and exclusively claim Abraham as their father alone, or that they alone represent Abraham's true faith (anathematizing all others). This is the same domestic rivalry that has existed in Abraham's household since the very beginning, akin to the feuds between Isaac and Ishmael, Hagar and Sarah, Jacob and Esau, Joseph and his brothers, etc. all jockeying for the sole blessing and birthright of Abraham, and seeking the other children be cast out of Abraham's household. Throughout history, these sibling rivalries have escalated to the brink of war again and again, extending to the political horizon of the world today. Despite the constant strife, rivalries and war in Abraham's household, his family has remained intact. Remember, Abraham grieved when Sarah had Hagar and Ishmael sent out of his household, but the result of this temporary expulsion was the reaffirmation that Hagar and Ishmael were included in God's promises to Abraham, and shortly afterwhich they were restored to Abraham's household. Remember, in the end, Abraham was buried by both Isaac and Ishmael (Gen 25:9).
When Christians deny Muslims worship the same God, I like to remind them that our common father Abraham was a stranger, sojourner, outlaw and Syrian immigrant (lit. "wandering Aramaean" Deut 26:5). So in a way, when we deny our Muslims are our brothers and sisters in the Abrahamic faith, simul. we deny that Abraham is our father and deny that we share the faith of Abraham, because by denying our brothers, we deny their father is our father. Another way to explain this is to remember that Islam is Christianity's younger brother just as Judaism is Christianity's older brother in Abraham Ecumene. A better way for Christians to respond to other Abrahamic faiths may be to seek unity with them, and invite them to have unity with us, as exemplified by Pope John XXXII when he met with Jewish Rabbis and said, "I am Joseph, your brother!" (Gen 45:4).
Abraham is not a common point of reference between Judaism, Christianity and Islam, but Abraham is a real starting point in the trialogue between these three Abrahamic religions. Hans Küng describes the term "Abraham Ecumene" to describe how Judaism, Christianity and Islam are united in their worship of the same God of Abraham, in his book, "Islam: Past, Present and Future". Küng's book is a tome, with many strengths and weaknesses, so this discussion is limited to Küng's chapter on Abraham. Abrahamic Ecumene is a rubric that acknowledges that Abraham is rightly claimed by Judaism, Christianity and Judaism, yet at the same time, recognizes that each of these three religions make exclusive claims about the life and faith of Abraham that is not easily harmonized with the other Abrahamic faiths. Abraham is not merely a "common point of reference" shared by Judaism, Christianity, and Islam, but Abraham is a "real starting place" between them.
Does that mean that Abraham represents ‘a common point of reference’ for the three religions only at first glance, while at a second glance ‘from the perspective of each religious tradition he is also the embodiment of what distinguishes them from one another and divides them’, so that Abraham can hardly be regarded as ‘an ideal starting point for present-day dialogue’? If we look more closely, Abraham does not necessarily appear to be an ideal starting point for what today can be called a ‘trialogue’ (a philological neologism) between Jews, Christians and Muslims. However, he is a real starting point. 
Abraham Ecumene recognizes that feuding and bloody rivalries exist within the family of Abraham. Küng argues that all this fighting, such as when Jews, Christians, and Muslims deny their brotherhood with the entire household of faith, has been going on since the beginning, and has yet to finally and totally divide the household of Abraham. Küng argues that the unity is visible, despite the conflict, by showing how Judaism, Christianity, and Islam are commonly opposed to eastern religions. Küng uses a river system metaphor to explain as follows:
What unites the religions of the Near Eastern river system beyond all the more or less chance historical relationships? What in principle unites Jews, Christians and Muslims? What can be regarded as the real foundation of an Abrahamic community which is emerging into consciousness and, given the independence of all three religions, has to be realized anew? What unites the three Abrahamic religions now? In inter-religious dialogues with Jews and Muslims one need only sit opposite representatives of the Indian and Chinese river systems to note how much is common to Jews, Christians and Muslims despite all the disputes: a largely similar basic understanding of God, human beings,the world and world history.
A fundamental and at the same time anticipatory conclusion is that Judaism, Christianity and Islam are linked by great common features associated with the name of Abraham: a kind of Abrahamic ecumene rooted in a long history, which hostility and wars could not obliterate. 
Abraham was a Henotheist, not a Jew, Christian or Muslim
Abraham ecumene unites Judaism, Christianity and Islam, but Abraham was not a Jew, Christian or Muslim. This is a key point to any Jew, Christian or Muslim, who denies that they worship the same god as the other Abrahamic faiths. Abraham was not a strict monotheistic Muslim, but he didn't affirm infallible or inerrant statements about Jesus being god either.
If we follow the book of Genesis, what is more important for Abraham is trust in God. Unconditional trusting faith is fundamental. It is said that this faith is ‘reckoned to Abraham for righteousness’. Throughout the Hebrew Bible faith (Hebrew aman—be firm; causative form he’emin—believe, trust) is never understood as acceptance of a truth which has been laid down, as holding the unprovable to be true, but as unshakable trust in a promise which cannot be realized in human terms: as faithfulness, as confidence, as saying ‘Amen’. Abraham is the prototype of someone who believes in this sense, a man who, on the basis of faith, can then withstand the greatest test: the sacrifice of his son Isaac, which is asked of him but in the end is not willed by God. 
Küng argues that Abraham was a henotheist who did not exclusively believe in one and only one God. Abraham had faith in one true God above all other gods, but he may have also believed in those other gods in a lesser degree. Küng argues that Abraham's conception of god showed commonality with the Canaanite God El, and that Abraham idea of god may have been formed by El. Additionally, Küng believes Abraham did not entirely abandon belief in the gods of his fathers, or other lesser gods entirely. Küng describes Abraham's faith in a way that is distinct from the later Jewish, Christian and Islamic descriptions of Abraham.
What kind of a God is being spoken of in these patriarchal narratives? From the beginning, the God of patriarchal religion was not bound either to heaven or to a sanctuary. He is the one ‘God of the father’ (the patriarch) to whom he has communicated his revelation: the God of Abraham, the God of Isaac, the God of Jacob, the God of the fathers. After the settlement this God took on elements of the Canaanite God El (under different names, such as ‘El Shaddai’), so that the God of Genesis can be described both as the God of the fathers and as El, and at the same time presents himself as a personal and a cosmic God. Thus today there is agreement among biblical critics that, like the lofty ethic of the Bible, its strict monotheism cannot have prevailed as early as the time of the patriarchs; from a historical perspective Abraham was certainly a henotheist who presupposed the existence of several gods but accepted only one God, his God, as the supreme and compelling authority. 
Abraham's Children Genesis: Ishmael and Isaac
Küng believes that the later Jewish writings in the Hebrew Bible and Christian writings in the New Testament decidedly align Abraham with the line of Isaac and Jacob. However, Küng also believes that the Genesis narratives do not likewise side with Jews or Christians, because he sees many elements in the Abrahamic narratives that exhibit an evenness between Isaac and Ishmael. Küng sources Karl-Josef Kuschel to provide a list of examples of how Isaac and Ishmael are described as equals in Genesis, contrary to the rests of the Hebrew Bible and New Testament:
Karl-Josef Kuschel is right in his book on Abraham when he works out precisely the positive statements about Ishmael in the interest of an Abrahamic ecumene:
1. Ishmael, not Isaac, was the firstborn son of Abraham (at the wish of his wife Sarah). Ishmael—‘God (hears)’.
2. Even before Isaac, Ishmael receives the sign of God’s covenant: circumcision.
3. Both Isaac’s survival and Ishmael’s survival are under God’s special protection. Ishmael’s rescue from the wilderness, narrated twice, corresponds to the rescue of Isaac from the threat of being sacrificed.
4. God’s promise of fertility and numerous descendants applies to both Isaac and Ishmael: ‘I will so greatly multiply your (Hagar’s) descendants that they cannot be counted for multitude.’ Like the sons of Jacob,Ishmael’s descendants form a group of twelve tribes. God explicitly says to Hagar: ‘As for Ishmael, I have heard you; behold, I will bless him and make him fruitful and multiply him exceedingly; he shall be the father of twelve princes,and I will make him a great nation.’
5. Not only Isaac but also Ishmael is present at Abraham’s burial:even though Hagar and Ishmael have been cast out into the wilderness, surprisingly Ishmael reappears at the death of his father Abraham: His sons Isaac and Ishmael buried him ...’ 
Judaism, Christianity and Islam form a three-strand-cord in the Abrahamic faith, and "Abraham Ecumene" explains how each of these three religions worship the same god of Abraham despite their conflicts. So even though Abraham is not "a common reference point" between them, he is "a real starting place" in the trialogue between Jews, Christians and Muslims. So whenever a Jew, Christian, or Muslim denies that they worship the same god as the other Abrahamic religions, or whenever a war occurs between Jewish, Christian and Muslim nations, then we can remember that is same sibling rivalry has has existed since the very beginning in Abraham's household, and has yet to destroy their common worship of the same God.
Many Jews, Christians and Muslims deny they worship the same god as the others, making exclusive claims about Jesus is God, or Allah and his prophet Muhammad, we can remember that Abraham's faith does not fit into any of these narrowly bounded definitions, and although he is the spiritual and physical father of all Abrahamic faiths, Abraham's faith continues to challenge all of the Abrahamic faiths today, and does not exclusively conform to any of these exclusive claims by Jews, Christians and Muslims alike. So in the end, let us pray and hope for unity in Abraham Ecumene, and hope that one day all people in all the Abrahamic Faiths would sit down together in Abraham's household and sing the children's song together: "Father abraham had many sons, you are one of them, and so am I . . ."
[^Header Image Background] "Isaac and Ishmael bury their father Abraham in the cave, as in Genesis 25:9: 'And his sons Isaac and Ishmael buried him in the cave of Machpelah, in the field of Ephron the son of Zophar the Hittite, which is before Mamre' (KJV); illustration from the 1728 Figures de la Bible; illustrated by Gerard Hoet (1648-1733) and others, and published by P. de Hondt in The Hague; image courtesy Bizzell Bible Collection, University of Oklahoma Libraries." By illustrators of the 1728 Figures de la Bible, Gerard Hoet (1648-1733) and others, published by P. de Hondt in The Hague in 1728 [Public domain], via Wikimedia Commons
[^1] Hüng, Hans. "A II. The Problems of the Beginning 3. Abraham--the Common Ancestor of the 'people of the Book'." Islam: Past, Present and Future. Oxford: Oneworld, 2009. N. pag. Print.