The Catechism in Questions & Answers

382. What do we mean when we talk about the “oneness of the church of Jesus Christ”?

The church is one because there is only the one God. The church testifies of the oneness of God, the Father, the Son, and the Holy Spirit, who works within it. Jesus specifically referred to the oneness of His followers and the love they bear one another as distinguishing features of those who belong to Him. It is in this way that the nature of God is manifest in the church: “God is love, and he who abides in love abides in God, and God in him” (1 John 4: 16).

Trinity: see Questions 61. et seq.