Certainly the idea of the Trinity contains some truth but it depends on how you define the component parts. Christians use the term “Son” in many inconsistent ways. Sometimes, it is just a synonym for Logos, the first property of God: His Word or Reason.
In this case it is easy to see how the Son is co-eternal with God, in the same way as the sun is inconceivable without its rays. Using this metaphor, we could speak of the Father and the Son, the sun and the ray, as being of the same substance light. At other times, however, the term “Son” refers simply to the man Jesus. In this case the Son serves as a synonym for the historical Jesus, the one who had a body and who suffered and died on the cross. This son was obviously not as physically great as the Father. Christians sometimes also use the term “Son” to refer to the nature of Jesus.
Obviously there are difficulties with casual use of the term ‘Son of God.’ To Muslims it threatens to trivialize the power and transcendence of God by ascribing to Him a human function having a son. As shown above, it also makes the historic Jesus a partner with God, ignoring His human and subservient aspects. As we shall see, Christian, Muslim, and Baha’i teachings all require that the humanity of Jesus and the humanity of each of the Chosen Ones of God be acknowledged. These Chosen Ones are, in Qur’anic terms, servants. The One true God can have no equal, no partner, and no son. Any attempt to introduce into the one Godhead any other is an attempt to ascribe the limitations of our world and language onto God’s. This Muslims correctly resist. Christians should too.
Also, a large part of this question of the Trinity pivots on logic and rationality. Most of the Christian theologians who have tried to explain the doctrine of the Trinity have failed. The prominent Christian philosopher Augustine, when he tried to explain the doctrine of the Trinity, ultimately concluded that an explanation is beyond human language, saying that “one must believe before one understands.” This appeal to faith rather than reason worked for some time during the medieval age, but as humanity progressed and education became more widespread, a pure appeal to accept a doctrine on faith alone began to lose its appeal.
After Augustine’s failure to describe the Trinity in logical or understandable terms, the great Christian theologian and thinker Thomas Aquinas concluded that both reason and faith must be requirements in theology; and he said, in his landmark work Summa Theologica, that the three-person theory of the Trinity could not be defended in any rational or understandable way:
It would seem that there are not several persons in God. For person is the individual substance of a rational nature. If then there are several persons in God, there must be several substances; which appears to be heretical.
The Baha’i view of the Trinity has a similar emphasis on the oneness of God, and it also defends rationality, asking that no one be required to accept a tenet of faith that can’t be rationally and logically understood:
The question of the Trinity, since the time of His Holiness Christ until now, is the belief of the Christians, and to the present time all the learned among them are perplexed and confounded. All have confessed that the question is beyond the grasp of reason, for three cannot become one, nor one three. To unite these is impossible; it is either one or three. – Abdu’l-Baha, Tablets of Abdu’l-Baha v3, p. 512.