Some religious groups that deny the doctrine of the Trinity include the Jehovah’s Witnesses and the United Pentecostal Church. These groups have been declared as being outside of Christianity due to their essentially non-Christian teachings. Christianity has always held to the Trinity and salvation by grace through faith, not of works. All who have ever denied the Trinity end up having some degree of good works which are needed for salvation. Further, denying the essential nature of God and creating another definition different from the scriptures is idolatry, and falls outside of Christianity.
One point that these heretical groups make is concerning the Bible passages that speak of Jesus as ‘only begotten Son.’ The following is an example is from The Oneness of God, by oneness pentecostal David Bernard:
John 3:16 calls Jesus the only begotten Son of God. . . The word begotten is a form of the verb beget, which means ‘to procreate, to father, to sire.’ Thus begotten indicates a definite point in time — the point at which conception takes place. By definition, the begetter (father) always must come before the begotten (offspring). There must be a time when the begetter exists and the begotten is not yet in existence, and there must be a point in time when the act of begetting occurs. Otherwise the word begotten has no meaning. So, the very words begotten and Son each contradict the word eternal as applied to the Son of God. (p.103-104)
The reasoning above has been given to me by Jehovah’s Witnesses as well. If this line of thinking is correct, then mainline Christianity has been a false teacher for many centuries, because we teach that Jesus is God the Son, and that the Son has existed eternally as the Son. We believe that Jesus is not the Father, but has lived in eternal relation in the Trinity, as one God, of three persons. The councils of Nicea (325 AD) and Chalcedon (451 AD) defined this doctrine.
However, the reasoning in the quote is incorrect. The confusion happens because these people have not carefully done their homework in regards to the meaning of the words.
In English, begotten and only begotten are quite similar. But the New Testament was not written in English, but in koine Greek. In Greek, the two terms are different words with different meanings. The word translated ‘begotten’ in our New Testament is geneo (strongs 1080), which means ‘to give birth to; to become the parent of.’ By contrast, the word translated ‘only begotten’ is monogenes (strongs 3439). It means “the only one of its kind; one and only; unique.” Any good Greek lexicon, such as BDAG, will give this information.
Thus a simple bit of language research could have prevented a heretical view of Christ. In any case, the Biblical teaching is clear: Jesus has existed eternally as the second person of the Trinity, and those who deny it are heretics that were, are, and will be outside of Christianity. All of the questions concerning the nature of Jesus were worked out in the early centuries of the church, and those who denied the orthodox position were cut off from the body of Christ. They will continue to be so.