Many non-Christian groups who accept some form of the New Testament - Muslims, Mormons, and Jehovah’s Witnesses – claim that the Christian doctrine of the Trinity is false. One of the arguments often used is that the concept of the Trinity cannot be found in Scripture, so the doctrine cannot be true.
This argument, however, will not fly. The argument for the tri-unity of God is straightforward and well grounded in the biblical text.
The second premise is that three persons - the Father, Son, and Holy Spirit - are all called God.
Concerning the Son, we read a verse such as John 8:58, where Jesus uses an Old Testament name of God, “I am,” to refer to Himself. We also read of instances where Jesus does things that only God would do, such as forgiving someone’s sins in Mark 2:5-7.
Concerning the Holy Spirit, we read a verse like Acts 5:3-4, where the Holy Spirit is called God.
Therefore, if the Bible teaches that God is one (and it does) and the Bible teaches that the Father, Son, and Holy Spirit are all God (and it does), then the doctrine of the Trinity is established. There are three persons in one God. This is what Christians affirm and non-Christians deny.
Now I have to quickly state that there are far more verses than the ones I mentioned above that establish the doctrine of the Trinity. I am only providing a tiny sampling in order to refute the claim that the Trinity is not based on the Bible. It is biblical, but please don’t think that my evidence above is exhaustive. If you want to dig deeper, then this study of the Trinity should more than satisfy you.