"We": the speaker is God Almighty, King of kings.
This expression is for glorification of His majesty, Lord of nations.
Prophet Abraham had his older son or first born son (Ishmael) from a slave woman: Agar.
Ishmael became the father of the Arab of Hijaz: Mecca and other cities.
And Abraham had the second son (Isaac) from his first woman, his cousin Sara.
Isaac became the father of the Israelite.
And has other sons from Qatoora, another woman whom he married after the death of Sara.
Jews, Christians claimed following Abraham in his monotheism and that he was Jewish or Christians, while in fact he was against idolatry.
So this is in the Quran 3: 67 about Abraham, which means:
([ Abraham in relation to Jews, Christians and Muslims: ]
[The reason for revealing the following aya:]
[The Jewish priests and the Najran Christians came to the messenger of God, and disputed concerning Abraham; the Jews said: he was a Jew, and the Christians said: he was a Christian; therefore, God revealed this aya:]
65. People of the Bible a, why do you dispute about Abraham [claiming he belonged to your religion], when the Torah and the Gospel were not revealed but only [a long time] after him b? Don't you realize c ?
66. So now, you [Jews] are those who [worshipped idols d in the past] argue about [Moses and Jesus: claiming that you follow their religion] that whereof you have knowledge.
But why do you argue about the matter [of Abraham] that you do not know;
God knows [the truth about the religion of Abraham] but you do not know [that e.]
67. Abraham was neither a Jew nor yet a Christian [as you claim], but was a "Hanief" [: exclusively devoting himself to God alone], a Muslim [: submitting himself to God], and was not one of the associaters f [: who associate idols or others with God in the worship.]
68. The people most worthy [to claim following the religion] of Abraham were his followers [at his time,] as are this prophet [Mohammed] and the believers [with him g]; God is the Protector of the believers h.)
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a i.e. Jews and Christians.
b And after revealing the Torah and the Gospel, the Judaism and Christianity appeared.
c The falsehood of your words.
d They worshipped the calf, the Baalim, Astaroth, the star Sirius and others, then you claim that Abraham followed your religion, while Abraham was not an associater or idolater, but he was a monotheist devoting himself to God alone, and he submitted himself to God alone.
e But only conjecture.
f while you associated and disobeyed.
g They are most worthy to say: "We follow the religion of Abraham"; because they worship God alone, and do not associate any creature with Him.
h He loves them and takes care about their affairs.