In the Old Testament there are only obscure indications of the Divine Mystery of the Holy Trinity. The Holy Bible says that before the creation of man, God said, Let us make man according to our image and likeness, which indicates that God is not one Person. But there is no indication as to how many persons are in the Godhead and who these conversers and co-creators are.
Elsewhere [in the Old Testament] the Divine Trinity is revealed somewhat more clearly, in the story of the Righteous Abraham, but again, only in the visible form of three angels.
In the New Testament the mystery of the Holy Trinity is manifest three times: at the Baptism, at the Transfiguration, and at the descent of the Holy Spirit
At the Baptism the Divine voice was heard: This is My Beloved Son, when the incarnate Son of God, the God-man, set out on His exploit of saving man. Here is the glory of God the Father, and His exultation at seeing this feat of love. God the Son stands in the streams of the Jordan in the form of a servant, while God the Holy Spirit, in the form of a dove, confirms the word of the Father, which testifies to the Divinity of the Son, Who humbly bows His head beneath the right hand of the Forerunner.
Sermon by St. John of Shanghai & San Francisco