The answer
The word "Trinity" is not mentioned in the Holy Bible, but the Holy Trinity is identified throughout the Holy Scripture: God the Father, God the Son, God the Holy Spirit, One God.The Old Testament taught the people the Law and how to ascertain the truth. Thus, three witnesses are essential to verifying and validating the truth.
The verse to which you are referring is in the New Testament Epistles—1 John 5:7. St. John the Evangelist and writer of this letter and two other letters, one of the four accounts of the Holy Gospel, and the Holy Book of Revelation, focuses his expositions on the Lord's divinity.
In 1 John 5:7, he emphasizes the established truth of the Holy Trinity, not only by three witnesses on earth, but also in heaven, and in God Himself in the Father, the Son, and the Holy Spirit, ONE God.
"Who is he who overcomes the world, but he who believes that Jesus is the Son of God?" (1 John 5:5).
Answered by His Eminence Metropolitan Youssef (opens in a new tab)