If all the people in the Bible were baptized in the name of Jesus, why does only mat 28.18 reads about the trinity which was a statement given Jesus himself.
I address this question in another question.
The short answer is that it was not original because it contradicts the many passages in Acts where they baptized in the name of Christ. In fact Paul's whole theology is built upon the notion of being joined to Christ:
[Rom 6:3 ESV] (3) Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death?
[Rom 6:8 ESV] (8) Now if we have died with Christ, we believe that we will also live with him.
[Col 2:20 ESV] (20) If with Christ you died to the elemental spirits of the world, why, as if you were still alive in the world, do you submit to regulations--
[Col 3:3 ESV] (3) For you have died, and your life is hidden with Christ in God.
Baptism into Christ is the warp and woof of Pauline theology.
Baptism into "the Trinity" is of Roman origin, not apostolic.