Christians do not understand their own book, the Bible, so good luck with teaching them the Quran! LOL.
4:159 Everyone among the People of the Book must believe before dying, in the Truth just stated. Jesus will be a witness against them on the Day of Resurrection [that he was neither crucified nor raised to the heavens. God is not confined to the heavens]
It is not the demise of Jesus [after his non return to earth] but those who believe in falsehood about him. They have a chance to redeem themselves while alive, to believe in the truth as expressed in the Quran about Jesus [normal birth, normal man, normal life, normal death], instead of the nonsense they ascribe to him [illegitimate, virgin birth, God, death on cross, salvation through his death on cross, return]. On Judgement Day, Jesus will be a witness that he did not say or do what they ascribe to him, and this occurs after we have all died and been raised again. When they are dead, their beliefs and deeds are fixed forever around their necks, either dragging them down to a hell, or floating up to a heaven [figuratively speaking]