DISCLAIMER: I do not endorse Sam Shamoun, do not validate Sam Shamoun's understanding of the Quran or the Muslim scholars he mentioned, and do not profess any competency in interpreting the Quran. My answer simply responds to what Sam wrote and to your question that arose from it.
Background of the quote
The quote came from a long article The Quran and the Holy Trinity by Sam Shamoun. The quote is the 5th point of Sam's response to what the 8th-century Muslim historian and hagiographer Ibn Ishaq and the medieval Muslim scholar al-Zamakhshari wrote.
According to Sam:
In fact, according to the earliest Muslim biographer Ibn Ishaq the Quran was specifically correcting this erroneous belief in God, Mary and Jesus as three separate divinities.
Another renowned Muslim commentator al-Zamakhshari candidly admitted that this passage was attacking the belief in Father, Mary and Jesus as three gods
According to Sam, early Muslims had bad information in the first place, as there were heretical groups that Christian mainstream themselves denounced:
The errors made by Ibn Ishaq and al-Zamakhshari regarding what Christians truly believe becomes apparent to anyone familiar with the basics of the Christian faith. In the first place, orthodox Christians have never taken Mary as a goddess alongside God. If the Quran were referring to a heretical group of Christians known as the Maryamites this would then serve to strengthen the position that these verses do not address Trinitarians, but apostates that deviated from the true faith. This also calls into question the whole episode of Ibn Ishaq’s report on the Christians from Najran and their beliefs. These Christians were either heretics, and if so then Ibn Ishaq is wrong for claiming that their belief that Jesus was the third of three is the doctrine of Christianity since these Christians do not speak on behalf of all of Christendom.
I refer you to the article to provide more background of the quote which may shed more light on his 5th point.
Answering your question
Now moving on to your question (edited slightly for clarity):
But don't Christians say God became Jesus regardless of how it is believed, whether in the form of the Triune God intervening into human history or in the form of Sabellianism, their outcome is the same: that God himself is Christ. In that context, isn't the Quran right that for Christians "God" is the Messiah? Also some Christians with Trinitarian understanding do believe God is Christ and Christ is God, i.e. they are one in essence. Then what is the problem?
Trinitarian concept as refinement of monotheism
God is ultimately a mystery that we cannot fully understand. On the other hand, Christians believe that God had revealed more of himself in Jesus, which gave us additional information that the early church fathers built into the Trinitarian concept. It's the nature of this additional information that the early church fathers and today's Christians defend so that false devotion, false practices, and false understanding don't follow.
Christians work out the concept of Trinity based on what is revealed to us by Jesus himself, which the apostles then recorded in the 27 books of NT for us, which were then reflected upon by the early church fathers. It's important to get this reflection process right, although only roughly. Throughout the reflection process in the first 3 centuries, monotheism was never compromised, but the Biblical data forced the orthodox (mainstream) early church fathers to reconcile Jesus's claim that he was the Son of God, and that the Holy Spirit was also God. They also needed to respond to heretical groups who distorted the authentic teaching of Jesus about himself and of his relationship with God the Father and with God the Holy Spirit such as:
- Sabellianism who said that the 3 members of the Trinity are merely modes (not full Person), or
- Arianism who said that the Son of God was begotten within time before Jesus was born instead of co-eternal with the Father.
Finally, they needed to work out a model that will explain how the incarnation saves us, human beings.
The Trinitarian conception and the Incarnation model that they came up with by the mid 5th century (see Trinity and Chalcedonian Definition) should be seen as a refinement of monotheism, not a repudiation or a distortion of monotheism. If early Muslims thought that Christians believe in 3 Gods, that means they misunderstood Christians a lot worse than the Sabellians or the Arians were, and that misunderstanding was reflected in the Quran.
"God is Jesus" ≠ "Jesus is God"
Sam is correct in saying that "Jesus is God" is not the same as saying "God is Jesus". This is NOT a bidirectional relation as in a mathematical equation where the expression J=G is the same as G=J. If this were the case, then what Jesus said, such as "my Father is greater than I" (John 14:28) or what Luke reported "In these days he went out to the mountain to pray, and all night he continued in prayer to God." (Luke 6:12), will not make any sense. This is why the early church fathers had to refine monotheism in light of Jesus's declaration that he is the Son of God.
The correct Trinitarian understanding is that we can relate to this one God in 3 ways, by relating to one of the three Persons of the Trinity at a time:
- God the Father, also known as the first person of the Trinity, or simply the Father
- God the Son, also known as the second person of the Trinity, or the Word
- God the Holy Spirit, also known as the third person of the Trinity, or simply the Spirit
while understanding that all Persons of the Trinity share a single essence.
But only God the Son added a human nature, incarnated into flesh so to speak, conceived as baby Jesus in the womb of Mary. It is EXTREMELY IMPORTANT that we don't understand this as "God became man". If we must use the word "became", the right expression is "The Word became human" (John 1:14 (NLT)) or more literally "The Word became flesh" (John 1:14 (ESV)) which after the Trinitarian conception was fully developed, this means "God the Son added a human nature" as Jesus. Thus Jesus fully retained his divinity as God the Son; he is one person with two natures. For a clear explanation, please watch this 11 minute video of Christian philosopher Eleonore Stump explaining what exactly "Jesus as God" mean.
When Christians speak of "Christ", we are referring to Jesus as "the anointed one" (al-Masīḥ). Christ / Messiah is simply a title and does not modify his Trinitarian relationship.
Why correct expression is important: because love has to be personal
Why is equating "Christ" with "God the Son" (second person of the Trinity) is important? Prayer and other acts of devotion in Christianity is deeply personal: person to person. One of the reasons why the early church fathers developed this concept of Person of the Trinity is to match the reality that in addition to previous revelation to Abraham, Moses, and the OT prophets, God finally revealed himself as a person in the flesh and blood of Jesus, a 100% human nature.
Why does this progressive revelation matter? Let's back up to the time before Abraham. Before Abraham, humankind had to grapple with glimpses of God through nature, deducing that God the creator must be all powerful, very creative, and demanding (because we have the moral law in our conscience). But is this God loving? Care for the humble and the weak and the oppressed? Give us eternal life after death? Close enough so we can pray to Him? The Israelites were given a big gift in God revealing "his character to Moses and his deeds to Israel" (Ps 103:7-18) so those who trusted in Him will receive blessings, forgiveness, and unfailing love. This means that God is no longer an abstract raw power to be feared, or a distant deity we cannot affect no matter what we do, or a capricious god that we need to placate without understanding (which is why there was a horrible religion which involves sacrificing children, see The Tragic History of Molech Child Sacrifice). The God of Israel revealed Himself sufficiently to the Israelites so we can know that if we obey His commandments and walk in His path God will be on our side. This is essentially the God of Judaism.
What did Christianity add to this knowledge of God? Christians believe that this God did a LOT more than simply revealing himself through prophets. He also demonstrated his love concretely and fulfilled his promises (including the promise in Ps 103:3-5) by revealing His innermost character in person (that is in Jesus). This adds great confidence to Christians in the trustworthiness and the goodness of this God. Although we still cannot fully understand him, Christians know God a lot more since in Jesus we can understand and relate to God as a person. Thus, when we pray to Christ, we pray to God the Son, the second person of the Trinity. When we want to understand God's love, we look at how Jesus loves us while he was on earth suffering for us. When we want to understand how to love others as God love them, we model ourselves after Jesus who forgave his enemies from the cross. When we are anxious about our upcoming death, we hope to be resurrected just as Jesus was resurrected from his death on the cross (which is why Christian belief that he truly died on the cross is critically important, see point #3 in my answer to another question).
Conclusion: why "God is Jesus" is problematic
I hope by now you begin to see "the problem". If we say "God is Jesus", this empties God of the other two Persons whom Christians relate to. Jesus himself taught us to call God of Abraham "Our Father" (see the Lord's Prayer), as he himself (in his human nature) prayed to the God of Israel by calling him "Father", a deeply personal relationship. With this refinement to monotheism, when we pray to God, it becomes a shorthand to pray to God the Father, who has adopted those who believed in Jesus as Jesus's spiritual brothers and sisters. BUT if we say "God is Jesus", we cannot relate to God as Father.
That is why Sam, along with all Trinitarian Christians (about 98%) are insistent that we don't say "God is Christ" or "God is Messiah", but "Christ is God" or "Jesus is God" (as the second person of the Trinity). But rather than saying "Jesus is God", Christians usually say "Jesus is the Son of God" or "Jesus, Son of God" to emphasize the Trinitarian relationship and to better reflect the Biblical wording (cf. Mark 1:1, Luke 1:35, John 1:34, etc.). Christians also use the phrase "Jesus is the Messiah" or "Jesus Christ" or "Christ of God" rather than "Christ is God", maybe to emphasize that it's the incarnated God in his human nature who is the Messiah (John 1:41, Acts 2:38, Matt 1:1, Luke 9:20).