It is not possible. The entire Epistle is about the controversy of the Jews and those who wanted to circumcise the Gentiles! How could they be ignorant of the Jewish religion when Paul is defending them from its abuses?
We might assume Paul told them at another time but we do not know, if he did not then it is false teaching to say some one is going to hell for believing in the trinity which is three.
How is that even logical? Even if your silliness was true, you would have us believe that the condemnation of being "accursed" would cease the moment they learned there is only one God, even though, under such instruction, it would still be part of the same message that Paul here makes clear must be followed.
It is not possible. The entire Epistle is about the controversy of the Jews and those who wanted to circumcise the Gentiles!
How would the gentiles know unless they were told that they could not believe as the Mormons do that there are three in the God head instead of one?
And there is no record of Paul telling them, shame shame for trying to add what you want to the scriptures.