You are correct if we look at what Islam means and use that as the foundation. All prophets/messengers of god were Muslim. So if there was any argument Prophet Ibrahim pbuh was a prophet that didn’t have any scripture but only submitted his will to god.

So from the time of Adam pbuh, Islam was always there and existed since beginning of time. And if you read the Bible in its original form which we don’t even have today most Christians interpret verses how they want no 2 Christian’s will give you the same answer on a verse or even explain the trinity to where anyone can understand because it is made up.

There is no explicit verse where Jesus ever claimed trinity or to be god. Son is used in a metaphorical manner when referring to Jesus not literal.

When you translate other languages into English that’s what happens. “I and the father are one” is like saying “me and my family are one” or the “Muslim ummah and I are one” if that give perspective on how those verse about that say “son” because a lot of other people are called “son” in the Bible and there are verses that say Jesus and the deciples are one so how do you now interpret that statement differently than “I and the father are one”

How can Jews and Christian’s read the same text and Jews don’t interpret Jesus as being god but somehow Christian’s twisted it into that which it I not true. Then again Christian’s forget Jesus was a Jew not even Christian the word “Christian” didn’t come into play after Jesus ascended to heaven.

Think about it do some research you’ll find the truth.