Trinity was mentioned in apostolic tradition
Saint athanasius of Alexandria said , his letter to serapion , chapter 28 : "us look at the very tradition, teaching, and faith of the Catholic Church from the beginning, which the Lord gave, the Apostles preached, and the Fathers kept. Upon this the Church is founded, and he who should fall away from it would not be a Christian, and should no longer be so called. There is, then, a Triad, holy and complete, confessed to be God in Father, Son, and Holy Spirit"
Saint Augustine of hippo said in his book about Trinity :" All those Catholic expounders of the divine Scriptures, both Old and New, whom I have been able to read, who have written before me concerning the Trinity, Who is God, have purposed to teach, according to the Scriptures, this doctrine, that the Father, and the Son, and the Holy Spirit intimate a divine unity of one and the same substance in an indivisible equality and therefore that they are not three Gods, but one God "
Saint Gregory wonderworker said "This is the perfect, holy, Apostolic faith of the holy God. Praise to the Holy Trinity for ever through the ages of the ages. Amen"
The Trinity was mentioned in apostolic tradtion and was understood, explained by the bible . The apostolic tradition is such as the liturgy of saint mark , written by st. Mark and mentioned Trinity . The liturgy of saint James also mentioned the Trinity . It was written by James , the brother of the lord .