From The Gospel of John, by Francis Martin and William Wright, commenting on John 16:12-15:
The Church prescribes this reading for Trinity Sunday. The Trinity has everything to do with all aspects of Christian life. The Catechism states, “God himself is an eternal exchange of love, Father, Son, and Holy Spirit, and he has destined us to share in that exchange” (221). This sharing is heaven. The Father sent his only Son to suffer, die, and rise, so that humanity could be restored to his friendship and enter into the divine communion. The Spirit has been sent to teach us, strengthen us, and help us replicate in our lives the same pattern of self-giving love that exists in God. Our task is to yield to the Holy Spirit, who makes the reality of God powerfully alive for us and draws us into communion with him through Jesus. In order to do this, we have to give up our sins and open ourselves up to God. The more attuned we become to the Holy Spirit by renouncing our sins and living a graced life of prayer and the sacraments, the more we will come to know the mystery of love that is the Blessed Trinity.