The word "Trinity" is not in the Bible, and the fully technical formula of "one God in three coequal, coeternal persons" is not stated there in a single verse. The stronger Christian case is not that the word appears, but that the doctrine is inferred from several passages read together.
The strongest form of the question
The objection is not a cheap word-search. Its best form is this: if the Trinity is the central truth about God, why do the prophets, Jesus, and the earliest preaching never state it plainly? The Bible repeatedly speaks of one God. Jesus prays to God, says he was sent by God, and distinguishes himself from the Father. So the critic asks: where does the Bible clearly teach that the Father, Son, and Holy Spirit are one divine essence, yet three distinct persons?
That is a serious question. A Muslim should not reduce it to "the word is absent," because many valid ideas are expressed without later technical labels. The real issue is whether the later doctrine is the natural meaning of the text, or a theological construction placed upon it.
How Christians usually build the case
The strongest Christian reply is that the Bible contains the "raw materials" of the Trinity. They point to verses affirming one God, then to verses where the Father is God, the Son is described in divine terms, and the Spirit acts personally and divinely. They also cite formulas where Father, Son, and Spirit are named together, such as baptism "in the name of the Father and of the Son and of the Holy Spirit" in Matthew 28:19, and Paul's blessing in 2 Corinthians 13:14.
They may also cite John's Gospel: "In the beginning was the Word, and the Word was with God, and the Word was God" (John 1:1), and Thomas saying to Jesus, "My Lord and my God" (John 20:28). From this angle, "Trinity" is a later word used to protect what they believe the Bible already implies.
What the argument proves - and what it does not
The absence of the word "Trinity" proves something modest but real: the Bible does not present the doctrine in the later creedal wording. The technical language of "persons," "essence," and coequality is the language of later church councils, especially Nicaea and Constantinople.
But absence of the word alone does not disprove the doctrine. A doctrine can be named later while being rooted earlier. So the fair test is not merely, "Is the word there?" The test is: "Is the doctrine clearly taught, or is it drawn by combining ambiguous texts against clearer ones?"
From the Islamic reading, the clearer line is prophetic monotheism: God is one, unique, not incarnate, not begotten, and not in need of partners within His being. The Quran addresses the issue directly:
Say, "He is Allah, One. Allah, the Eternal Refuge. He neither begets nor is born, nor is there to Him any equivalent."
Quran 112:1-4
So believe in Allah and His messengers. And do not say, "Three"; desist - it is better for you. Indeed, Allah is but one God. Exalted is He above having a son.
Quran 4:171
Why Muslims remain unconvinced
Muslims see Jesus as a mighty messenger and Messiah, born miraculously, supported by the Holy Spirit, and honored above most of creation. But worship belongs to God alone. When Jesus prays, obeys, is sent, and calls people to the One who sent him, that fits prophethood more naturally than coequal deity.
So the objection establishes that the Trinity is not plainly named or formally defined in the Bible. It does not, by itself, refute every Christian attempt to infer it. But it does show why Muslims regard the doctrine as a later theological reading, not the simple message of Jesus and the prophets: worship the One God without partner.