"The Lord, the Lord, a God gracious and merciful, slow to anger and abounding in love and fidelity." (Exodus 34: 6 NABRE).
Who is God? Above Scripture verse is the self-revelation of God.
Today is the Solemnity of the Most Holy Trinity: the eternal, holy and undivided Unity. Three Persons in One God. A most profound mystery. Although I do not understand a bit of this, I believe it absolutely because Jesus Christ revealed it and the Church teaches it as truth.
"Glory to the Father, the Son, and the Holy Spirit; to God who is, who was, and who is to come." (Cf Revelation 1: 8)
"The grace of the Lord Jesus Christ and the love of God and the fellowship of the holy Spirit be with all of you." (2 Corinthians 13: 13).