If the People of the Book rely upon Abraham, let them study his history. His posterity included both Israel and Isma'il. Abraham was a righteous man of God, a Muslim, and so were his children. Abraham and Isma'il built the Ka'ba as the house of God, and purified it, to be a center of worship for all the world: for God is the God of all Peoples.

God of All Peoples [2:122-141]

Children of Israel, remember My blessing where with I blessed you, and that I have preferred you above all beings; and beware a day when no soul for another shall give satisfaction, and no counterpoise shall be accepted from it, nor any intercession shall be profitable to it, neither shall they be helped.

And when his Lord tested Abraham with certain words, and he fulfilled them. He said, 'Behold, I make you a leader for the people.' Said he, 'And of my seed?' He said, 'My covenant shall not reach the evildoers.' 

And when We appointed the House to be a place of visitation for the people, and a sanctuary, and: 'Take to yourselves Abraham's station for a place of prayer.' And We made covenant with Abraham and Ishmael: 'Purify My House for those that shall go about it and those that cleave to it, to those who bow and prostrate themselves.' 

And when Abraham said, 'My Lord, make this a land secure, and provide its people with fruits, such of them as believe in God and the Last Day.' He said, 'And whoso disbelieves, to him I shall give enjoyment a little, then I shall compel him to the chastisement of the Fire -- how evil a homecoming!' 

And when Abraham, and Ishmael with him, raised up the foundations of the House: 'Our Lord, receive this from us; Thou art the All-hearing, the All-knowing; and, our Lord, make us submissive to Thee, and of our seed a nation submissive to Thee; and show us our holy rites, and turn towards us; surely Thou turnest, and art All-compassionate; and, our Lord, do Thou send among them a Messenger, one of them, who shall recite to them Thy signs, and teach them the Book and the Wisdom, and purify them; Thou art the All-mighty, the All-wise.' 

Who therefore shrinks from the religion of Abraham, except he is foolish-minded? Indeed, WE chose him in the present world, and in the world to come he shall be among the righteous. When his Lord said to him, 'Surrender,' he said, 'I have surrendered me to the Lord of all Being.' And Abraham charged his sons with this and Jacob likewise: 'My sons, God has chosen for you the religion; see that you die not save in surrender.' 

Why, were you witnesses, when death came to Jacob? When he said to his sons, 'What will you serve after me?' They said, 'We will serve thy God and the God of thy fathers Abraham, Ishmael and Isaac, One God; to Him we surrender.' 

That is a nation that has passed away; there awaits them that they have earned, and there awaits you that you have earned; you shall not be questioned concerning the things they did.

And they say, 'Be Jews or Christians and you shall be guided.' Say thou: 'Nay, rather the creed of Abraham, a man of pure faith; he was no idolater.' Say you: 'We believe in God, and in that which has been sent down on us and sent down on Abraham, Ishmael, Isaac and Jacob, and the Tribes, and that which was given to Moses and Jesus and the Prophets, of their Lord; we make no division between any of them, and to Him we surrender.' 

And if they believe in the like of that you believe in, then they are truly guided; but if they turn away, then they are clearly in schism; God will suffice you for them; He is the All-hearing, the All-knowing; the baptism of God; and who is there that baptizes fairer than God? Him we are serving. 

Say: 'Would you then dispute with us concerning God, who is our Lord and your Lord? Our deeds belong to us, and to you belong your deeds; Him we serve sincerely. Or do you say, "Abraham, Ishmael, Isaac and Jacob, and the Tribes -- they were Jews, or they were Christians"?' Say: 'Have you then greater knowledge, or God? And who does greater evil than he who conceals a testimony received from God? And God is not heedless of the things you do.' 

That is a nation that has passed away; there awaits them that they have earned, and there awaits you that you have earned; you shall not be questioned concerning the things they did.