The trinity makes NO sense. The trinity states that God exists in 3 entities.!
More like the other way around, three entities exist in God. You can believe in oneness if you want to, but I'll stick to the definitive judgment of the Church.
Polycarp (70-155/160 A.D.). Bishop of Smyrna. Disciple of John the Apostle.
"O Lord God almighty... I bless you and glorify you through the eternal and heavenly high priest Jesus Christ, your beloved Son, through whom be glory to you, with Him and the Holy Spirit, both now and forever" (n. 14, ed. Funk; PG 5.1040).
Justin Martyr (100?-165? A.D.). He was a Christian apologist and martyr.
"For, in the name of God, the Father and Lord of the universe, and of our Savior Jesus Christ, and of the Holy Spirit, they then receive the washing with water" (First Apol., LXI).
Tertullian (160-215 A.D.)
"We define that there are two, the Father and the Son, and three with the Holy Spirit, and this number is made by the pattern of salvation... [which] brings about unity in trinity, interrelating the three, the Father, the Son, and the Holy Spirit. They are three, not in dignity, but in degree, not in substance but in form, not in power but in kind. They are of one substance and power, because there is one God from whom these degrees, forms and kinds devolve in the name of Father, Son and Holy Spirit." (Adv. Prax. 23; PL 2.156-7).
Furthermore the Nicene Creed which was declared to be binding dogma by the Council of Nicaea in A.D. 325 in opposition to the Arians says thus:
We believe in one God,
the Father, the Almighty
maker of heaven and earth,
of all that is, seen and unseen.
We believe in one Lord, Jesus Christ,
the only Son of God,
eternally begotten of the Father,
God from God, Light from Light,
true God from true God,
begotten, not made,
of one Being with the Father.
Through him all things were made.
For us men and for our salvation
he came down from heaven:
by the power of the Holy Spirit
he became incarnate from the Virgin Mary, and was made man.
For our sake he was crucified under Pontius Pilate;
he suffered death and was buried.
On the third day he rose again
in accordance with the Scriptures;
he ascended into heaven
and is seated at the right hand of the Father.
He will come again in glory to judge the living and the dead,
and his kingdom will have no end
We believe in the Holy Spirit, the Lord, the giver of Life,
who proceeds from the Father and the Son.
With the Father and the Son he is worshiped and glorified.
He has spoken through the Prophets.
We believe in one holy catholic and apostolic Church.
We acknowledge one baptism for the forgiveness of sins.
We look for the resurrection of the dead,
and the life of the world to come. Amen.
The oneness movement is basically Arianism. It is just a old heresy with a new name. But, the following reasons are why i believe in the Trinity. The divine teaching authority of the Church has proclaimed it so and the scriptures testify to it. If i believed anything else i would be anathema.