The Koran says Abraham discovered the Muslim faith, crediting him with calling it “Muslim.”

And strive hard in (the way of) Allah, (such) a striving as is due to Him; He has chosen you and has not laid upon you an hardship in religion; the faith of your father Ibrahim; He named you Muslims before and in this, that the Messenger may be a bearer of witness to you, and you may be bearers of witness to the people; therefore keep up prayer and pay the poor-rate and hold fast by Allah; He is your Guardian; how excellent the Guardian and how excellent the Helper! Surah 22:78

The Koran says that Abraham’s faith was the true faith.

O People of the Scripture! Why will ye argue about Abraham, when the Torah and the Gospel were not revealed till after him? Have ye then no sense? Lo! ye are those who argue about that whereof ye have some knowledge: Why then argue ye concerning that whereof ye have no knowledge? Allah knoweth. Ye know not. Abraham was not a Jew, nor yet a Christian; but he was an upright man who had surrendered (to Allah), and he was not of the idolaters. Lo! those of mankind who have the best claim to Abraham are those who followed him, and this Prophet and those who believe (with him); and Allah is the Protecting Guardian of the believers. Surah 3:65-68

And this was the legacy that Abraham left to his sons, and so did Jacob; “Oh my sons! Allah hath chosen the Faith for you; then die not except in the Faith of Islam.” Were ye witnesses when death appeared before Jacob? Behold, he said to his sons: “What will ye worship after me?” They said: “We shall worship Thy god and the god of thy fathers, of Abraham, Isma’il and Isaac,- the one (True) Allah: To Him we bow (in Islam).” Surah 2:132-133

Say (O Muslims): We believe in Allah and that which is revealed unto us and that which was revealed unto Abraham, and Ishmael, and Isaac, and Jacob, and the tribes, and that which Moses and Jesus received, and that which the prophets received from their Lord. We make no distinction between any of them, and unto Him we have surrendered. Surah 2:136

It is true that Abraham was not Jew or Christian. It is probably true that he was surrounded by idolaters since that practice was rampant during his time.

It is also true that Abraham preceded the Torah and Gospel, but he also predated the Koran. He was neither Jew nor Christian. . .nor Muslim.

In fact, it would have been impossible for him to term the Arabic phrase “Muslim” since there are no known texts of the language around during the time of Abraham.

ARABIC LANGUAGE: The earliest surviving texts in Proto-Arabic, or Ancient North Arabian, are the Hasaean inscriptions of eastern Saudi Arabia, from the 8th century BC, written not in the modern Arabic alphabet, nor in its Nabataean ancestor, but in variants of the epigraphic South Arabian musnad. These are followed by 6th-century BC Lihyanite texts from southeastern Saudi Arabia and the Thamudic texts found throughout Arabia and the Sinai, and not actually connected with Thamud. (Wikipedia)

Abraham was around approximately 2000 BC but the Arabic language doesn’t emerge until 800 BC.

The Muslim faith that Abraham believed, the Koran says, he passed down to Ishmael, Isaac, Jacob, the tribes, Moses and Jesus.

The Bible agrees that Abraham had a true faith in God that was passed down over the ages. However, the Bible shows that his trust in God was the foundation of the Jewish faith (his grandson was named Israel) and the Christian faith (Matthew 1 and Luke 3 show that Jesus descended from him).

Abraham’s faith, says the Koran, was not the same as the People of the Book.

Not your desires, nor those of the People of the Book (can prevail): whoever works evil, will be requited accordingly. Nor will he find, besides Allah, any protector or helper. If any do deeds of righteousness,- be they male or female - and have faith, they will enter Heaven, and not the least injustice will be done to them. Who can be better in religion than one who submits his whole self to Allah, does good, and follows the way of Abraham the true in Faith? For Allah did take Abraham for a friend. Surah 4:123-125

This verse in Surah 4 groups together Christians (People of the Book) with people who do evil. The true faith of Abraham, it says, is one who do “deeds of righteousness” AND have faith. It’s a works based righteousness.

Then the word of the LORD came to him: “This man will not be your heir, but a son coming from your own body will be your heir.” He took him outside and said, “Look up at the heavens and count the stars—if indeed you can count them.” Then he said to him, “So shall your offspring be.” Abram believed the LORD, and he credited it to him as righteousness. Genesis 15:4-6

However, in Genesis 15 from the Bible it is very clear that Abraham’s righteousness came about by simply believing in God’s promise. He performed no deeds but simply looked up to the sky and saw the stars, which represented the vast number of descendants to come from him.

No works. Just faith. That’s Abraham’s true faith.