"The grace of the Lord Jesus Christ and the love of God and the fellowship of the Holy Spirit be with you all." 2 Corinthians 13:14
A. There are three persons in the Godhead; the Father, the Son, and the Holy Ghost; and these three are one God, the same in substance, equal in power and glory.
The Athanasian Creed, an early ecumenical statement within church history, makes this bold statement:
"Whosoever will be saved, before all things it is necessary that he hold the catholic faith;
Which faith except every one do keep whole and undefiled, without doubt he shall perish everlastingly.
And the catholic faith is this: That we worship one God in Trinity, and Trinity in Unity."
God has revealed Himself in Holy Scripture as the God who eternally exists as 3 persons: God the Father, God the Son, and God the Spirit.
But is this necessary to know for salvation? Isn't just believing in Jesus enough? No.
To confess Jesus as the Son of God, and Lord, is to confess He has a Father, and no one can call Christ Lord except by the Spirit (1 Corinthians 12).
Salvation clings to our Triune God, for it is the Father who loves and chooses His people, Jesus Christ the Son who ransoms His people through His life, death and resurrection, and the Holy Spirit applying Christ's work to His people through the gift of faith and repentance (Ephesians 1:3-14).
Though God revealed this Truth to His people, it was in shadows of the Old Testament, yet fully revealed through Jesus Christ in the New Testament.
The Benediction of Numbers 6 reads, "The LORD bless you and keep you; the LORD make his face to shine upon you and be gracious to you; the LORD lift up his countenance upon you and give you peace," finds its Triune fulfillment in Paul's benediction in 2 Corinthians 13. "The Lord make his face shine upon you" is the love of the Father; "The Lord be gracious to you" is the grace of the Lord Jesus Christ; "The Lord lift up His countenance upon you and give you peace" comes from the fellowship of the Holy Spirit.
The Holy Trinity is not a mathematical conundrum to be described by eggs and water, but the God whom we must love and trust and obey for salvation.