If that were the case, there couldn't be any doubt.
Why is there differences of opinion, and why were ecumenical councils held to establish orthodox doctrine, if it is in the Gospels?
It is NOT explicitly mentioned ANYWHERE in the Bible, that I'm aware of.
..the developed doctrine of the Trinity is not explicit in the books that constitute the New Testament..
...
Reflection by early Christians on passages such as the Great Commission: "Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit" and Paul the Apostle's blessing: "The grace of the Lord Jesus Christ and the love of God and the fellowship of the Holy Spirit be with you all", leading theologians across history in attempting to articulate the relationship between the Father, Son, and Holy Spirit. Eventually, the diverse references to God, Jesus, and the Spirit found in the New Testament were brought together to form the doctrine of the Trinity—one Godhead subsisting in three persons and one substance. The doctrine of the Trinity was used to oppose alternative views of how the three are related and to defend the church against charges of worshiping two or three gods.
-wiki Trinity-
I know you will say that the doctrine of the trinity is eternal, but this is not borne out by the evidence.
There are many different creeds that existed in early Christianity,
and the argument of "Church Fathers" is not convincing.
So much evidence was destroyed over the first few centuries, that what little evidence remains is what the authorities WANTED to remain.
The Nag Hammedi library and dead-sea scrolls along with similar finds have provided modern historical scholars with extra evidence to show that medieval beliefs were misplaced, and were the agenda of the political establishment.
Many people are frightened of change, and would rather stick to convention, than have an open mind and embrace some "foreign" religion.
Of course, Christianity is originally a "foreign" religion, but it was established so long ago, that it is ingrained in western culture.