The Catechism in Questions & Answers

64. What references to the trinity of God are recorded in the New Testament?

When Jesus, the Son of God, was baptised in the Jordan, heaven opened and the Holy Spirit descended upon Him like a dove. From heaven, the Father attested: “You are My beloved Son, in whom I am well pleased” (Mark 1: 10- 11). The Father, the Son, and the Holy Spirit were thus present together.
The three divine persons are also mentioned in the Great Commission which Jesus issued to His Apostles: “All authority has been given to Me in heaven and on earth. Go therefore and make disciples of all the nations, baptising them in the name of the Father and of the Son and of the Holy Spirit” (Matthew 28: 18-19).
The wording of the benediction recorded in 2 Corinthians 13: 14 likewise refers to the trinity of God: “The grace of the Lord Jesus Christ, and the love of God [the Father], and the communion of the Holy Spirit be with you all!”