♦ Jesus said, “My Father is greater than I.” (John 14:28) This directly contradicts the Trinitarian doctrine which says, “the Godhead of the Father, of the Son, and of the Holy Ghost, is all one; the Glory equal.” (Athanasian Creed)
♦ Jesus prayed, “Father, if thou be willing, remove this cup from me: nevertheless not my will, but thine, be done.” (Luke 22:42) Jesus wouldn’t have distinguished between His own will and the Father’s in his prayer if He and His Father were the same personage. Besides, if they were literally one, He would not have prayed that way to Himself.♦ Jesus told Mary at the empty tomb, “I am not yet ascended to my Father: but go to my brethren, and say unto them, I ascend unto my Father, and your Father.” If Jesus and His Father were literally one personage, Jesus would have already been in His own presence and could have lingered longer to console the bewildered Mary.
♦ Jesus said, “The Son will judge but not the Father.” (John 5:22) If Jesus and His Father were the same person, this sentence would make no sense. It is logically impossible for God to both judge and not judge.
♦ Jesus said, “All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men.” (Matt 12:31) So, blasphemy against the Father and the Son is OK but not against the Holy Ghost when they’re all literally one person? That illogical.
♦ Jesus said, “Hereafter shall the Son of man sit on the right hand of the power of God.” How can God sit on His own right hand?
Among the last words of Jesus on the cross were, “My God, my God, why hast thou forsaken me?” (Matt 27:46) If Jesus and His Father were literally one person, why would He have asked Himself why He forsook Himself?
Why would God send Himself to be the Son of Himself? Even traditional Christian artists depict the godhead as three:
There are many additional scriptures that tell of a godhead that is three personages. For more scriptures on this topic and some information on the origins of the Trinitarian doctrine, see: Rich Wilkinson's answer to Which Christian denominations do not believe in the Trinity? Why not?
Occam’s Razor! The simplest answer tends to be the right one. Trinitarian doctrine is impossible to understand or explain, though many have tried. Some like to use this graphic to explicate the mystery of a triune God:
However, just drawing something doesn’t prove it’s a possible reality. For example, the Penrose Triangle is easy to draw, but impossible to build:
By contrast, the following event involving three separate divine individuals is easy to translate into reality:
♦ Jesus being baptized in the Jordan.
♦ His Father speaking from heaven, “This is my beloved Son.”
♦ The Holy Ghost descending from heaven with the sign of the dove.
(Matt 3:16–17)
If all three were literally one, why act like three? Some ventriloquism would have been needed.
If you invited God to dinner, how many additional place settings would you need?