While John's Gospel is the summit of the revelation of the Holy Spirit in the New Testament, the letters of St. John also provide valuable insight into the work of the Holy Spirit. Also, Revelation, the last book of the Bible, is an apocalyptic work that focuses on the prophetic inspiration of its author and ends with the symbolism of the Spirit as the river of life-giving water in the heavenly Jerusalem. The epilogue of the Apocalypse declares: "The Spirit and the Bride say 'Come'... And let him who is thirsty come, let all who desires take the water of life without price" (Rev 22:17).

Although there are not any "new" revelations of the Holy Spirit in these New Testament epistles, they are significant because the Holy Spirit is spoken of in various ways. Especially in the letters of St. Peter, we see echoes of revelations of the Holy Spirit's work from the writings of St. Paul and the Gospels.

The Holy Spirit is the ultimate source of unity in the Church and in the world, being the love within the Trinity and the source of love that binds people together and overcomes sinful divisions. St. Paul emphasizes the Spirit is the "love of God poured into our hearts" (Rom. 5:5). He urges all in the Church to "be eager to maintain the unity of the Spirit in the bond of peace" (Eph. 4:3). Also highlighted are a 1990 and 1993 catechesis of Pope St. John Paul II on the Holy Spirit and unity, and Vatican II's teaching on the Holy Spirit and the quest for the unity of Christians, or ecumenism.