God replied, "Look up to the heavens and count the stars. . . . So shall be your seed."

On that day, Genesis says, God made a covenant with Abraham: "To your seed I have given this land from the river of Egypt to the great river, the river Euphrates."

From Salem, Abraham went to Mamre and Hebron, where he now spent most of his time. I visualize him as a grand old man, sitting under a tree, dispensing wisdom, overseeing the family finances, and, of course, talking with God.

At this point Genesis records an event that would profoundly influence the course of world history. In the ancient Middle East wives who could not bear children encouraged their husbands to procreate with slaves or concubines. Thus Sarah, who was barren, convinced Abraham to have a child with Hagar, an Egyptian slave who had probably stayed with them since the clan's expulsion by the pharaoh.

The birth of Ishmael, Abraham's first son, foreshadowed the emergence in Arabia in the seventh century a.d. of a new religion—Islam—under the guidance of the Prophet Muhammad. The Koran calls Abraham's first son an apostle (and) a prophet ... He was most acceptable in the sight of his Lord. Ishmael's pedigree lent legitimacy to the new faith, but the Koran never mentions Hagar's name.

Abraham first, then Ishmael, are the perfect models of piety for Muslims. Abraham's name appears in 25 of the 114 chapters of the Koran, and to this day Ibrahim and Ismail are common first names among Muslims. "The Koran explains that all true revelations come from God," says John Voll, professor of Islamic history at Georgetown University's Center for Muslim-Christian Understanding. "It is the record of the divine revelation, which is shared by all the scriptures."

There is no doubt that Muhammad and his inner circle of disciples believed in Abraham as the founder of their faith. The Koran orders Muslims to follow the religion of Abraham. Abraham was not a Jew nor yet a Christian; but he was true in Faith, . . . and he joined not gods with God.

Muhammad was born in Mecca around 570. There he was surrounded by Jewish and Christian communities—although Muslims do not believe that these faiths influenced the revelation of Islam. In 622 Muhammad moved to Medina, where his following quickly grew. He was recognized as the last in a series of prophets, including Adam, Abraham, Moses, and Jesus, all of whom appear, redefined, in the Holy Book of Islam.

The Koran reports that Abraham and Isma'il raised the foundations of the House. The "house" is the Kaaba in Mecca, Islam's holiest shrine. One of the four corners of this small rectangular structure is a sacred black stone that is a remnant of the original building. The annual pilgrimage to Mecca, the hajj, when Muslims from all over the world circle the Kaaba, reinforces the central role of Abraham and Ishmael in the Islamic faith.

The Koran does not give particulars about the birth of Ishmael, but Genesis goes into great detail. It reports that after Hagar became pregnant, Sarah resented her. She complained to Abraham that when the Egyptian "saw she had conceived, I became slight in her eyes," and she went on harassing the girl. Abraham replied meekly, "Look, your slavegirl is in your hands. Do to her whatever you think right."

Consequently Hagar fled from Sarah into the desert wilderness. Sarah's motivations are blurred, but what intrigues Rabbi Adin Steinsaltz is that she acted independently of Abraham when circumstances required. As the rabbi put it, Sarah and Abraham were as much partners as a married couple, and "she would allow Hagar to be an instrument of procreation but would not allow her the honor and privilege of being Abraham's beloved wife-companion." By law, Steinsaltz said, "women were quite independent. They had the right to own property, and they had standing. Sarah had a say, in one way or another." I asked him if this makes Sarah the first great feminist. "Yes," the rabbi shot back.

God, for his part, took another view of the situation. An angel intercepted Hagar when, apparently heading home to Egypt, pregnant, she stopped at a spring near Kadesh in the Negev. Hagar told the messenger she was fleeing from Sarah, but the angel ordered her to "return to your mistress and suffer harassment at her hand." As a consolation the angel said to Hagar, "Look, you have conceived and will bear a son and you will call his name Ishmael for the Lord has heeded your suffering." Hagar obeyed. Ishmael (whose name in Hebrew means "God has heard") was born. Abraham was said to be 86 at the time.

Thirteen years after Ishmael's birth the 99-year-old Abraham was summoned by God, who made explicit his choice of Abraham as the father to a multitude of nations. To symbolize the significance of this new, exalted status, God changed his name from Abram to Abraham. God also changed the name of his wife, Sarai, to Sarah. Then God announced that "I will also give you from her a son," and upon hearing this, Abraham flung himself on his face and he laughed, saying to himself, "To a hundred-year-old will a child be born, will ninety-year-old Sarah give birth?"

In their next meeting, God appeared to Abraham when he was sitting outside his tent. Looking up, Abraham saw three travelers among the trees. In a customary display of hospitality to strangers, he fetched water to wash their feet and treated the visitors to curds and milk and a calf he had cooked. Waiting on them as they ate (the scene depicted in Rembrandt's famous etching "Abraham Entertaining the Angels," owned by the National Gallery of Art in Washington, D.C.), he heard God repeat the promise that Sarah would have a son. Sarah, who had been listening from inside the tent, laughed inwardly, expressing her doubts. "After being shriveled, shall I have pleasure, and my husband is old?...Shall I really give birth, old as I am?"

After playing host at Mamre, Abraham moved from Hebron back to Beersheba. Within a year his son Isaac ("he who laughs" in Hebrew) was born. Abraham circumcised him on the eighth day, in keeping with God's order that every male be circumcised.

Genesis then speaks of a second expulsion of Hagar. Sarah demanded this after observing the much older Ishmael playing and laughing with Isaac; she wanted to assure Isaac's inheritance, even though he was not the firstborn. Now God took Sarah's side, ordering Abraham to send Hagar and Ishmael away. He told him that "through Isaac shall your seed be acclaimed. But the slavegirl's son, too, I will make a nation, for he is your seed."

Hagar and her son were banished to the desert, but they were not alone. God provided for them, giving them a well of water when Hagar had lost all hope. Ishmael, Genesis says, grew up and dwelled in the wilderness, and he became a seasoned bowman. The Bible reveals little else except that his mother procured him an Egyptian wife and he helped bury his father. This is the last mention of Hagar. Muslim tradition holds that mother and son stayed together in Mecca, and they are said to be buried in a common grave—Hijr Ismail—next to the Kaaba.

Accompanied by Avner Goren, I followed Abraham to Beersheba. When we stopped at one Bedouin settlement, children rushed forward to beg: for water, not money. Abraham, too, needed water, and he dug a well in Beersheba, hoping to live in peace with the local inhabitants. He also planted a tamarisk tree, a symbol of plenty, invoking the name of the Lord, everlasting God. At this stage I envision Abraham as a full-time proselytizer and one-God activist.

The day of our visit to Beersheba was unusually raw; the Negev had just had more than half a foot of snow—one of the heaviest snowfalls in 50 years—and the whitened palm trees looked festive and beautiful. Beersheba was the patriarch's home for a number of years. A well said to be the one dug by Abraham still exists in the center of town, just off busy Hebron Road. (But it no longer provides water.)

Recognizing the city's spiritual importance, in 1979 Anwar Sadat, then president of Egypt, and Menachem Begin, the Israeli prime minister, came to Beersheba to begin peace negotiations between their two nations. But as Goren and I stood in the snow at Abraham's well, three Israeli Air Force F-16 fighter-bombers roared overhead. The message was plain: The Middle East is still far from real peace. Achieving it, repairing Abraham's fractured spiritual legacy, will demand an extreme act of faith from Palestinians and Israelis, whose common heritage is now a matter of scientific proof. A recent study of the DNA of male Jews and Middle Eastern Arabs—among them Syrians, Palestinians, and Lebanese—shows that they share a common set of ancestors.

The ultimate test of Abraham's faith in the only God appears to have arisen in Beersheba, when God ordered Abraham to take Isaac to the land of Moriah and offer him up as a burnt offering on one of the mountains.

When Abraham and Isaac reached their destination—which Jewish and Christian tradition holds to have been the Temple Mount in Jerusalem, the site today of the Dome of the Rock shrine—the patriarch erected an altar. He bound Isaac and placed him on a pile of wood on the altar. But when Abraham raised the cleaver to kill his son, God's messenger called out from the heavens, "Do not reach out your hand against the lad, and do nothing to him, for now I know that you fear God." A ram, caught by its horns in a nearby thicket, was presented as a burnt offering instead of Isaac.

In the Koran, God similarly tests Abraham's faith by ordering the sacrifice of his son, but the son and the place are not named. In sura, or chapter, 37:102, 112 Abraham said, "O my son! I see in vision that I offer thee in sacrifice." When Abraham shows his willingness to comply with God, he is promised another son, Isaac. And We gave him the good news of Isaac—a prophet,—one of the Righteous. Most Muslims therefore believe that Ishmael was the one to be sacrificed and that this test occurred in or near Mecca.

In Genesis, Abraham returned to Beersheba. Sarah died in Qiryat Arba, near Hebron, at the age of 127. Abraham buried her in the Cave of Machpelah, in a tomb he bought for 400 silver shekels. He then dispatched a servant to the city of Nahor in northern Mesopotamia, near Haran, to find a wife for Isaac. Rebekah was the chosen woman. Back in Hebron again, Abraham had to be the busiest old man in all of Canaan. He found himself a new wife—a woman named Keturah, who gave him six children.

Abraham died at the ripe old age of 175. Isaac and Ishmael buried him in the Machpelah cave next to Sarah.

In a sense Abraham never died. On the highest religious level Abraham and his monotheism was a model for Jesus and his early Christian disciples and, much later, Muhammad and his Muslim followers. Today he still stands out as a unique spiritual figure, transcending the frontiers of great religions. However questionable the accuracy of the scriptures, however thin the archaeological and historical evidence, Jews, Christians, and Muslims still revere him as the patriarch.

One of the most touching expressions of devotion to Abraham I encountered on my travels was a short poem, "Hymn to the Blessing of Abraham," given to me at Istanbul Technical University. It was written by a Muslim, Cengizhan Mutlu, and tells of King Nimrod, who plotted to kill Abraham for his monotheism. My Turkish guide, Aydin Kudu, provided an impromptu translation.

Idol made of pure gold

Gives no hope, no food.

Nimrod doesn't comprehend this.

Wood burns (for the stake),

Smoke reaches the sky,

Ibrahim is thrown into the fire.

He feels no pain, he doesn't groan.

He says, "My God will save me."

Two angels had said it rightly.

Embers turn into ashes,

Sparkles turn into roses.

"My God will save me." In these five simple words is the essence of Abraham and his astonishing endeavors. They spell out his fundamental belief that there is one God. That belief changed the world forever.