The Catechism in Questions & Answers

306. What is the significance of the First Commandment in the New Testament?

The First Commandment states that there is only one God. This is the triune God: Father, Son, and Holy Spirit. In the New Testament, the First Commandment is not only applied to God, the Father, but rather also to Jesus Christ and the Holy Spirit.

Trinity: see Questions 61. et seq.