I gave the context
10:90 And We took the Children of Israel across the sea, and Pharaoh and his soldiers pursued them in tyranny and enmity until, when drowning overtook him, he said, "I believe that there is no deity except that in whom the Children of Israel believe, and I am of the Muslims." <<<Muslim meaning one who submits to GOD.
91: Now? And you had disobeyed [Him] before and were of the corrupters?
92: So today We will save you in body that you may be to those who succeed you a sign. And indeed, many among the people, of Our signs, are heedless
93: And We had certainty settled the Children of Israel in an agreeable settlement and provided them with good things. And they did not differ until [after] knowledge had come to them. Indeed, your Lord will judge between them on the Day of Resurrection concerning that over which they used to differ
So now you have the context verse 94 makes sense:
94: So if you are in doubt, [O Muhammad], about that which We have revealed to you, then ask those who have been reading the Scripture before you. The truth has certainly come to you from your Lord, so never be among the doubters.
If you doubt these verses regarding the story of Moses and the Israelites, ask the people of the book for confirmation.
Early Church didn't accept this letter as it appeared forged. None the less it's wormed its way into the NT. The term for servant is Dulos, which also means 'Slave' :
James 1:1
James, a slave of God and of the Lord Jesus Christ: To the 12 tribes in the Dispersion. Greetings.
I see no problem here, James pbuh is affiming that like his brother he too is a slave of God. He calls his brother Lord Messiah. I accept Jesus is a son of God as understood by first Century Jews, meaning pious God fearing person.
The Church in Jerusalem didn't teach trinity, that word isn't in the Bible. James pbuh preached Torah, fasted and prayed tirelessly to GOD alone, he also rejected the made up Gospel of grace as preached by Paul.