The statement is made by Muslims and Christians both.
“Christians believe in a Triune God.”
I have never seen this. What I see is A God who provided a way to bring humanity into a full communion with Him. He created the life in the human Jesus to serve in the capacity of revealing His Perfect Will, as it would be done through human flesh on earth, so that we would no longer be stumbling in errant ideas and would be clear about what God wills for us as human beings on earth. He provided the continuation of the promise and glory of this perfect human example in the resurrection which would reveal to us the eternal nature of Christ, and allow us to receive the Spirit of His Truth (God’s) til the end of days on earth, proving that there was no further need for a ‘prophet’, since all who believe on what God had given humanity through the person of Christ would receive His Spirit of Love and Truth, as Christ was fully present and revealing to us in the flesh, in our own hearts minds and souls, through our faith in Him alone.
The trinity is the way that God designed for humanity to draw closer to Him, to know more about His perfect truth, to commune with Holy truth and to be able to discern this from what humanity will falsely claim to be God’s “Word”.
That Muslims in particular have such a great distortion of this relationship that God created with humanity through Jesus as the vehicle and the Holy Spirit as the continual presence for future generations to come to Him, that they call it a division of God himself, instead of an action of God that is defined as “triune” in purpose and function can not fully rest on the errors of Muhammad, but in the wording even Christians use when they attempted to capture what Jesus did indeed teach. One should ask, what does this ‘trinity’ bring? What are its implications? Did Jesus at any time say, as Muslims falsely believe, “God is 3”? Or did Jesus simply continue to tell humanity what he had been sent to reveal, that God was present in Him, to show them the true way to love and worship, to demonstrate in actual actions in the flesh what this would look like were we all to fully submit only to God’s will, and to promise them that even after his perfect example in the flesh was taken from the planet, there would remain the light of His Truth to be bestowed upon all that had accepted Gods invitation to dwell with His Love in the flesh on earth, as Christ had, by the virtue of the GIFT of the Holy Spirit to be given unto believers.
And this gift is how we discern what ‘truth’ is, it is what guides us to understand false teachings, Jesus life is what we have to compare the claims of other prophets against, in that He did no thing which was not Gods will, which is why we see no wars, no violence, no taking of slaves, no multiple wives. To say that God would not do such a thing, to create a life for us to truly be able to SEE in action, that would be the expression of Gods will alone, so that we could not call unholy acts done by previous (or future) prophets holy acts, so that we could know what God is truly about and could come into that glorious joyful UNION with Him through is own Holy Spirit filling our hearts and souls, something Islam is completely empty of, is to deny God’s immense love and mercy for His creation in humanity.
We needed what God gave in Christ, for we were all led astray by errant teachings, misinterpretations of the Law and lacked the perfect human example that only God could provide to humanity. We as humans need to see and touch and have an example to follow before we could begin to trust in the deeper and more powerful truth that God revealed in Christ. The Holy Spirit is that deeper truth, that God dwells in every heart that seeks Him through Christ and this fulfills what Jesus proclaimed, that we too would do the works that he did and even more (for his body would be layed down and he would be leaving the fleshly life on earth), for we would be able to receive what he had been sent to show to humanity, the true power of God, which is LOVE and nothing more, not political rule, not violent oppression, and this power is given by virtue of the Holy Spirit, that which one can not see but can know as it reminds us of the life and Word of Christ, when He was sent to glorify God and be a light unto all nations and people that they would languish in dark error no more.
This is the purpose of the Trinity.
God is not a ‘Triune’ God, God is one, whole, never ending power that can not be captured with human definitions. But what God did was present a Triune path for humanity to be reconciled back to His Light, as Adam was in the Garden when he could fully commune with God, without the aid of another human telling him what God said, he was not separated by anything until the fall. It is Jesus who provided the Way back to God, as we live in the flesh, we can experience the constant companionship that Adam enjoyed, we can hear Gods own voice, not the voice of scholars who have so many opposing views and false rules and beliefs about Gods will.
We are each reconciled back to God by the work God did in Jesus through the granting of the Holy Spirit.
If you seek God in any other way, you will not get there, you will only be deceived by the false understandings of men who were driven by more than a pure will which only Jesus was created with in humanity.
This is the Way, the Truth and the Life. God, Jesus and the Holy Spirit, this creates the reunion of humanity in the flesh to God by virtue of their heart, mind and soul.