Regarding the Quran (Surah al-Ma'idah) 5:17
They have certainly disbelieved who say that Allāh is Christ, the son of Mary. Say, "Then who could prevent Allāh at all if He had intended to destroy Christ, the son of Mary, or his mother or everyone on the earth?" And to Allāh belongs the dominion of the heavens and the earth and whatever is between them. He creates what He wills, and Allāh is over all things competent.
A Christian missionary wrote
Fifth, the Quran also distorts Christian beliefs regarding the Person of Christ when it accuses Christians of saying that God, or Allah, is the Christ. The historic Christian position is that Jesus is God, which is not the same as saying that God is Jesus. The former implies that Christ is fully God in essence; that he has the entire essential attributes of Deity and is all that God is, whereas the latter suggests that Christ is the only one that is God. In other words, saying that God is Jesus means that the entire Godhead is instantiated in Christ alone to the exclusion of the Father and the Holy Spirit, or that the Father, Son and Holy Spirit are nothing more than manifestations of a single Divine Person. This latter understanding would imply that Christ is all three manifestations, which is a form of an ancient heresy known as Sabellianism which was condemned by the early Church, why would it still be the problem for the Quran
But don't the Christians say God became Jesus, regardless of how it is believed whether in the form of Triune God intervening into human history or Sabellianism (their outcome is same that God himself is Christ in that context)? Isn’t the quran right that Christians believe "God" is the Messiah? AlsAlso, some Christians in trinity do believe God is Christ and Christ is God i.e. they are one in essence. Then what's the problem?