According to Jesus...
According to Jesus, God is strictly one Person, not three. Christians who value Jesus as the supreme revealer of truth should consider his classic words, uttered in a final prayer: "You, Father, are the only one who is truly God" (John 17:3). He defined salvation as belief in that one and only true God, and in himself as the Messiah (John 17:3).
It is a serious hijacking of the words of Jesus if one adds to Jesus' creed. For Jesus, his Father is "the one who alone is truly God," "the only one who is truly God," "the one true God" (see also John 5:44 and Mark 12:29).
Those utterances are more than clear. They are without a hint of ambiguity. Yet they have been abandoned by the church bearing the name of Jesus. The church has for centuries, since post-biblical times, defined God as three Persons. Jesus defined God as one Person, the Father. There is a very great difference. That difference calls for extensive rethinking and reform. We cannot risk fragmenting God. Jesus believed and taught strict unitary monotheism. He had never heard of the Trinity -- or if he had he rejected it. So should his followers.
According to Augustine...
Centuries later, after church councils had invented iron-clad creeds and imposed them on the faithful, Augustine came face to face with Jesus' definition of God as the "only one who is truly God." What was he to do? The church by then had lost Jesus' creed. It propagated everywhere belief that God was three Persons. That innocent sentence in John 17:3 stated that God was a single Person, not three Persons. Here is Augustine's "solution." He wrote: "The proper order of the words is, 'That they may know You and Jesus Christ, whom You have sent, as the only true God'" (Tractates on the Gospel of John, 105).
One can have the words of Jesus on this great issue, or the words of the post-biblical creeds. It is hard to see how one can have both at the same time. Following Jesus means believing his teachings. Jesus' teaching about how many Persons are the One God is really not difficult: "You, Father, are the only true God." Jesus is the Lord Messiah (Luke 2:11; Ps. 110:1), the Son of God (Matt. 16:16), but not the one true God. The word "one" should be clear to all.
According to God
If anyone has any question about this, let him consult the thousands upon thousands of singular personal pronouns used for God in the Bible. "I," "Me," "Mine," "Myself," "Thee," "Thy," "Thine," "Thyself," "He," "Him," "His," "Himself." All these words, as well as God's proper name YHVH followed by singular verbs (6700 times), ought to convince the open-minded that God is one Person, not more. And monotheism — belief that God is one — is, according to Jesus, of critical importance (Mark 12:29).
Jesus, the Son of God, is the perfect human reflection of the One God, his Father. But he is not God. He is the sinless second Adam and the "prophet to be raised up from the house of Israel" (Deut. 18:15-18). Created and begotten in the womb of his mother under the power of God's spirit, he is designated "Son of God" (Luke 1:35). The idea that he is "eternally begotten" not only has no recognizable meaning in language, but it is false to Scripture. "Eternal generation" contradicts the important biblical fact that the Son of God was begotten "today," not in eternity (Ps. 2:7; cp. Acts 13:33, referring in the latter text to the birth of Jesus).