With mere human reason it is impossible. But with God’s revelation of Himself we can know things. For example, Man could have never known God’s name without Him revealing it to Man. So it’s a question of God’s self-revelation. We know about the Trinity because of what God has revealed about Himself.
The three divine Persons are only one God because they equally possesses the fullness of the one and indivisible divine nature. Thus theTorah begins with referring to God in the plural Elohim, not because God is many parts or many Gods, but because of the true nature of God, namely the Mystery of the Holy Trinity. They are distinct from each other by reason of the relations which place them in correspondence to each other:
The Father generates the Son; the Son is generated by the Father; the Holy Spirit proceeds from the Father and the Son.
We know this through God’s revelation of Himself. Christ fulfilled all the prophecies, performed countless miracles and showed authority over nature and life itself, thus the unfolding revelation of God to Man culminated with Jesus Christ.
The Eternal Word (the Son of God) became flesh and assumed a true humanity. Thus Christ’s physical body was finite; at the same time, in the body of Jesus we see God made visible. No longer do we relate to God as an abstraction but as someone who proved His love for us, someone who makes us inheritors of His Eternal Kingdom, someone whom we can know and love with our human affections and have a share in His Eternal Life.
The Eternal Word of God assumed a human nature in order to accomplish our salvation. As God and author of creation itself, the unique event of the Incarnation of the Eternal Word does not mean that Jesus Christ is part God and part man, nor does it mean that he is the result of a mix of the divine and the human.
The Eternal Word became truly man while remaining truly God. Thus Jesus Christ is true God and true man, thus to worship the Eternal Word of God is to worship God Himself.
Thus Jesus said “Anyone who has seen me has seen the Father” . . “I am in the Father, and that the Father is in me” . . “If you love me, keep my commandments. And I will ask the Father, and he will give you another advocate to help you and be with you forever—the Spirit of truth.”
God’s unfolding revelation of Himself culminated with Himself taking a human nature not only to dwell among us and teach us the Truth about Himself, but to take on a human nature in order to suffer for our sins on the cross as The True Lamb of God.