Kafir is a person who does not believe in God. Trinity is blasphemy against God. Qur'aan does not describe Trinity in detail. It refutes it and condemns it. It also confirms that Jesus is not the son of God because God has no son.Here we go...
The Trinity is not "Kafir". First of all, the Quran doesn't describe the Trinity correctly. Secondly the entire persecution of Jews happens and has happened under Islamic rule, where many were forced to convert.
If Jews are not Kafir, what then does Muhammad offer to them that they should "convert" to?
What makes you say that the Qur'aan does not describe Trinity correctly?
The persecution and killing of Jews was carried out only by Christianity over centuries during her darkest history. If there were no Islam, the Jews would have become extinct at the hands of Christianity.
Muhammad had only wanted the Jews of his time to believe that he was the Prophet of God, promised to them.